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|“||We are brothers, detectives of the NYPD, and strong believers of employing the scientific method in dissecting crime scenes. Due to our modern methods, we are sidelined by our colleagues and overlooked by the police force. But little do they know, our approach will soon be the driving force of our field, and us teaming up with Dr. Kreizler in catching this murderer will prove our technique to be effective.||”|
|— Isaacson brothers|
He's a main character portrayed by Douglas Smith.
"DETECTIVE SGT. MARCUS ISAACSON is an expert in the criminal sciences and forensic medicine. Both he and his twin brother Lucius are outcasts within the New York police department, partly because they are Jewish and partly because of their progressive and scientific approaches to crime solving. Marcus fancies himself a ladies’ man and is the more outgoing of the fraternal twins. The pair are called upon to help with the investigation."
Appearance and Personality Edit
Detective Sergeant Marcus Isaacson is a handsome young man, about six feet tall. He has curly brown hair, hazel eyes, and a fair complexion. His clothes are always impeccably clean and tidy, although they are not stylish as those of John Moore and Dr. Kreizler. Contrary to his twin brother Lucius, Marcus is a sort of dandy and playboy that takes great care of his appearance with perfumes and the like, especially if invited to important, or fancy events. Not very attentive to the precepts of the Torah, Marcus surrenders moderately to pleasures like any man of his age – even defining himself halfway between man and beast, because he surrenders to his natural sexual desires towards women – and this causes quite a few frictions between the two brothers. Marcus is also very extrovert and easy-going and a recurring event is to present himself as Detective Sergeant and Lucius just as his brother; However, it is unclear whether Marcus does this deliberately or not.
Kreizler Institute, 1896. Detective Sergeant Marcus Isaacson presented himself and "his brother, Lucius" to Dr. Kreizler and John Moore, much to Lucius's annoyance for not being formally introducing with the respect his position demands. When Dr. Kreizler asked for how long he could have enjoyed the benefits of their expertise, Marcus answered all the time he wanted, since he doubted they would have been missed at the police department. Dr. Kreizler, then, speculated if the ostracism suffered at the Police Department's was due to their modern methods of investigation. Marcus nodded, but Lucius added that the fact that they were Jews also influenced that. Sent by Commissioner Roosevelt to assist Dr. Kreizler in his inquiries, both brothers showed themselves skilled and professional in their work, often finding themselves finishing each other's sentences. Marcus recalled the Zweig murder case in great detail, interrupting Dr. Kreizler before he could proceed with a briefing about them. Given the advanced decomposition state of Benjamin Zweig's corpse, Marcus was dubious about Kreizler's expectations of finding similarities with Giorgio Santorelli's mutilation. 
Marcus and Lucius were in a butchery, raging with different types of blades on a cow's severed head, trying to determine what kind of blade had produced the wounds on the body of Benjamin Zweig and allegedly also on that of Giorgio Santorelli. Lucius was frenzied as usual, asking Marcus to let him try too. In response, Marcus asked the butcher another knife, ignoring Lucius' requests. When Marcus' attention was directed to a young woman delivering flyers on the street in front of the butchery, Lucius hit him on the shoulder with his fist, implicitly suggesting his brother to pay attention to their work. Having found similar knife-marks produced by the last blade tasted on the head's orbital cavities, they bought it.
Coming out of the butcher's shop, Marcus took one of the flyers posted by the girl on a pylon. After taking a look at the content, Lucius sarcastically asked Marcus if he was interested in socialism now. Marcus replied if he knew the difference between capitalism and socialism. When Lucius said he didn't, Marcus explained that "in capitalism, man exploits man. In socialism, it's the other way around."
In the evening, Marcus and Lucius were preparing to leave while Mrs. Isaacson painted fake roses in the dim light of their house. Talking in Yiddish, she asked where they were going since Marcus was using too much scent. Marcus said they had police business to attend, and Mrs. Isaacson replied that they did not seem to be going to do police business. Lucius stated that their mother intended to say that he "stinks like a 10-cent whore", and Marcus, annoyed by his brother's joke, wondered how Lucius could know what smell a prostitute has. While Marcus arranged to turn off most of the candles and lamps, Lucius reminded their mami to turn off the candles before going to sleep. She protested, saying it was Shabbat but Lucius replied that it was not that night.
At Delmonico's, Marcus and Lucius looked around, astonished for such an ambiance, not being accustomed to such a luxury. Lucius prevented one of the valets from taking his doctor's bag. They reached the room booked by Dr. Laszlo Kreizler, where they met Miss Sara Howard. As usual, Marcus introduced himself as Detective Sergeant, introducing Lucius simply as his brother. Lucius corrected him, then asked the woman if he was there to take notes for Commissioner Roosevelt. Shortly after, they were joined by Dr. Kreizler and John Schuyler Moore, taking place around the table. Kreizler proposed a toast to a fruitful partnership, and Lucius told his brother that he did not believe the wine was Kashrut. While the waiters began serving the first course, John Moore asked the reason for the meeting and Marcus stepped in before Kreizler had the opportunity to explain himself. Marcus and Lucius took turns in their explanations about their recent discoveries, showing the diners the weapon bought by the butcher, an Arkansas Toothpick, specifying how such a blade perfectly reflected every prerequisite required by the wounds inflicted on the corpses. They then passed to another discovery, introducing the concept of Dactyloscopy – the science of a finger, palm, or foot leaving a chance impression – and helped by Miss Howard to explain to Moore and Kreizler what it consisted of since she had done some personal reading on the subject. While Marcus showed a magnifying glass on a support containing his and Lucius's thumbs' finger-marks, the latter showed a timepiece found on the burial suit of Benjamin Zweig, bearing a bloody fingermark almost presumably left by the murderer.
At the end of dinner, Sara and the Isaacsons made their way to the exit, chatting about their common interests in the forensic sciences and modern investigative techniques, leaving Kreizler and Moore behind to argue.
Instead of going home with his brother, however, Marcus went to one of the Socialist Labor Party meetings he had read about on the flyer that morning. While one of the Socialist Labor Party spokesmen stirred up the crowds during the meeting, Marcus took a pint of beer, passing strategically in front of the girl of the flyers, greeting her and exchanging a series of glances with her during the rally. Afterward, the two had sex in a rented room. During the frenzy of the sexual intercourse, Marcus asked the girl to remember him her name; She introduced herself as Esther, saying that she was pleased to meet him. He replied, "I'm Marcus, likewise." 
Marcus was arranging some equipment on behalf of Dr. Kreizler with his brother Lucius, while the doctor and Miss Howard questioned John Moore about his whereabouts the night before in Tenderloin. Listening to John's story about the previous night, Marcus ironically commented that he did not have to worry because such situations would happen even to the best of them.
Some time later, the Isaacson brothers and Dr. Kreizler analyzed the fingerprints found on the timepiece projecting it onto a screen using a modern machine. When Dr. Kreizler pointed out a mark, Lucius suggested it could be a partial print but on closer inspection, the three men agreed that it was a scar or other kind of sign on the murderer's finger. Interested in ascertaining whether or not it was a misprint or a real scar, they decided to analyze the Santorelli boy's corpse in search of other finger-marks.
When they reached the morgue, Marcus and Lucius briefly conversed with Mr. Tuthill. In particular, Lucius claimed that Roosevelt was a man thirsty for reform instead of a "bunch of hooey" as stated by Mr. Tuthill. The three men then moved Giorgio Santorelli's coffin on one of the operating tables, only to discover an empty coffin.
Late at night, Marcus and his brother were analyzing the body of another child killed on the roof of the old immigration station, soon to be modernized as an aquarium. While the Isaacson brothers were photographing the crime scene, the rest of the team expressed opinions and conjectures about how the murder had taken place. The arrival of Captain Connor and the journalists prompted the team to leave the crime scene, having used more than enough the extra time given them by Roosevelt. 
Memorable Quotes Edit
- Marcus Isaacson (to Dr. Kreizler and John Moore): "I find the bones to be the most reliable witnesses while my brother is more a "man of the flesh"..."
- Lucius Isaacson: "He means the soft tissue!"
- — ‘The Alienist’ Overview 
- Lucius Isaacson: "Thinking about becoming a socialist now, are you?"
- Marcus Isaacson: "You know the difference between capitalism and socialism? In capitalism, man exploits man. In socialism, it's the other way around."
- Mrs. Isaacson: "Vau geystu" (Transl. from Yiddish: Were are you going?)
- Marcus: "We have police business."
- Mrs. Isaacson: "Es tut nisht shmekn vi politsay business" (It doesn't smell like police business.)
- Lucius: "She means you stink like a 10-cent whore..."
- Marcus: "How would you know?"
- — A Fruitful Partnership
- Rosie: "Buy us a bottle of champagne?"
- Marcus: "How old are you?"
- Rosie: "How old would you like?"
- Marcus: "What's your name?"
- Rosie: "Rosie. Take me upstairs."
- Marcus: "How about some poetry instead, Rosie? A pansy by way of Khartoum took a lesbian up to his room. They argued all night as to who had the right to do what, with which, and to whom."
- — Ascension
- In the novel, Marcus trained as a lawyer before becoming a detective, whilst Lucius attended medical school. Before being recruited by Theodore Roosevelt as Sergeants Detectives for the New York City Police Department, they briefly worked for the Pinkerton Detective Agency.
- In the novel, Marcus and Lucius have a younger sister, Cordelia, named after a character in King Lear, a tragedy by William Shakespeare.
- According to writer and executive producer Hossein Amini, the Isaacson brothers are "Jewish detectives being sidelines by the generally Irish Catholics police force of the time." Director and executive producer Jakob Verbruggen added: "Through the eyes of the Isaacsons, we witness the birth of forensics."
- Douglas Smith revealed that he became a close friend with Matthew Shear, the actor who portrays his brother Lucius in the TV series. They built their friendship by researching their characters, watching documentaries together, reading novels and manuals of the time to better understand the mindset of the Isaacson brothers. 
- Smith also said that Lucius reminds him of his own older brother, actor and director Gregory Smith, so he "kind of plugged [his] older brother into Lucius’ role and plugged [himself] into Marcus’ role." 
- Smith spend a spend a lot of time in the Tenement District on Hester Street, the neighborhood of New York where the Isaacson family lives.