Editing Mark Greene (section)

==Character illness and death==
While suturing a patient's wound, Greene loses control of his faculties and is temporarily unable to speak. After a CT scan and a biopsy, he is diagnosed with an aggressive form of brain cancer, [[glioblastoma multiforme]], that is thought to be inoperable. Embarrassed, Greene briefly tries to hide his condition, but his cover is blown when he has a seizure while arguing with Carter.

With little time, Greene seeks a second opinion from an eminent New York City neurosurgeon, Dr. Burke. Greene is told that the tumor is near a critical section of his brain but hasn't "invaded" it yet and they can perform an operation on New Year's Eve 2000. Greene's surgery is performed by Burke and things appear to be positive, although it takes him a while to return to his old self. A year or so later, however, Greene finds out his tumor has returned, and Dr. Burke both confirms this and says he cannot operate again because the tumor regrowth is now in part of his brain where an operation would render Greene completely vegetative.  While chemotherapy treatment will only allow Greene to live for another 5–6 months, Burke points out: "You should have been dead a year ago, Mark.  You got married, saw your daughter be born - I'd say that was time well spent."

At this point, Rachel has run away from Jen in St. Louis and is staying with Mark and Elizabeth.  Though she vehemently denies it, her [[recreational drug use]] becomes apparent when her baby sister Ella gets hold of some [[Methylenedioxymethamphetamine|ecstasy]] in her backpack and nearly dies after ingesting it in the episode "Damage is Done".

When Rachel shows up, Mark can barely control his anger at her, berating her for repeatedly for lying to him and for putting Ella in danger. However, he sees her remorse and fear for Ella are genuine. Knowing that Elizabeth is angry enough for both of them, he hugs Rachel when she starts to cry. When Mark refuses to throw Rachel out of the house, Elizabeth says she won't return home with Ella as long as Rachel is there and leaves home with Ella and moves into a hotel. Unwilling to tell Elizabeth about his condition, Mark stays with Susan during the course of his chemotherapy and radiation treatments. Elizabeth later finds out the truth and wants to come home, but Mark tells her she shouldn't pretend to be his wife just because he's sick. However, she returns anyway and begins helping Mark as his terminal illness advances. Mark suffers more medical problems and his time in the ER comes to an end.

Eventually, however, he resigns himself to his fate and decides to halt the debilitating chemotherapy, deciding he would rather have three good final months than twice that suffering from his treatments.

On his last day in the ER, he meets with the same older woman that viewers saw on the first episode of ''ER''. She again has a hangnail and complains about how painful it is. Greene tells her that he has an inoperable tumor, asks another doctor to treat her, and tells the patient not to return to the ER again. He leaves the ER, stops his chemotherapy treatments, tells [[John Carter (ER)|John Carter]] that ''he'' will now "set the tone" and takes Rachel on a last-minute trip to Hawaii to rebuild his relationship with her and relive happier times.

After several moves around the island and some conflict with a surly Rachel, Mark suffers from increased symptoms, including a seizure while walking with Rachel, prompting her to call Elizabeth, who comes to Hawaii with Ella. One night, Rachel comes to her father's room while he sleeps. Mark awakens and smiles at Rachel, telling her with slurred speech that he was just dreaming of her and how she used to love balloons. He tells her that he was trying to think of a piece of advice that every father should tell his daughter, and tells her to be generous with her time, her love, and her life. Rachel tells her father that she remembers a lullaby that Mark used to sing her when she was a baby and slips a pair of headphones on his head and plays [[Israel Kamakawiwo'ole]]'s rendition of "[[Over The Rainbow]]" for him as he smiles and falls back asleep. While the song plays, he is seen walking through an empty ER. The next morning, Elizabeth discovers that he has died.

In the episode "The Letter", Carter discovers two faxes that had arrived earlier, both sent by Elizabeth. He reads the first one to the staff. It is a letter that Greene had written about his sentiments about the Chicago County ER where he had worked for many years, and the staff he had worked with throughout the years. They begin happily discussing his letter until Lewis notices that Carter is holding the second fax, visibly upset. He informs them that the second is a brief letter written by Elizabeth, notifying them that Mark had died that morning around 6 A.M., "… at sunrise, his favorite time of the day."

She explains that she sent the first letter to show the staff at the ER what he thought of them. As the staff responds sadly to this news, Frank asks if he should post the second fax on the staff bulletin board, and Carter tells him to post both of them. The letter and the news of his passing send many of the ER's staff that day into emotional turmoil, with Kerry Weaver going from second-guessing Abby's posting of the letter (she quickly changes her mind and says it should stay up) to crying and stating her regrets to Sandy Lopez that she's lost a friend. This portion of the episode closely models the scene at the end of the film ''[[Mister Roberts (1955 film)|Mister Roberts]]'', when [[Jack Lemmon|Jack Lemmon's]] character, Ensign Pulver, reads two similar letters connected with the title character's death. It was also revealed in the episode by [[Susan Lewis]] that he died at the age of 38, making his birth year-end 1963 or 1964.

At the close of the episode, as the staff rush out to the ambulance bay to handle incoming casualties, the wind blowing through the open door tears one page of Greene's letter off the bulletin board.

Greene's body is returned to Chicago, where he is buried. Many of his friends and colleagues come to the funeral: [[John Carter (ER)|John Carter]], [[Peter Benton]], [[Kerry Weaver]], [[Abby Lockhart]], [[Luka Kovač]], [[Susan Lewis]], [[Jing-Mei "Deb" Chen|Jing-Mei Chen]], [[Robert Romano (ER)|Robert Romano]], [[Jerry Markovic]], [[List of secondary nurses in ER#Lydia Wright|Lydia Wright]], [[Frank Martin (ER character)|Frank Martin]], [[List of secondary doctors in ER#Donald Anspaugh|Donald Anspaugh]], William "Wild Willy" Swift (played by [[Michael Ironside]] in 1994), [[Haleh Adams]], [[Michael Gallant]], [[Cleo Finch]], Jen, Rachel, Ella, and [[Elizabeth Corday|Elizabeth]].

After the funeral, Rachel asks Elizabeth if she can visit to see Ella; Elizabeth responds "Of course, she's your sister." Rachel then suddenly asks the driver to pull over: she walks to a bunch of balloons attached to a fence, takes a purple one, and slowly lets go of it, watching it rise toward the sky.

Rachel goes back to living with her mother in St. Louis but later returns to Chicago when the time comes to select a college, as well as asking a bemused Elizabeth to help her acquire effective birth control pills.

In the April 2009 ''ER'' series finale, Rachel returns to County General to interview for a position as a med student, showing that she has become a responsible young woman and followed in Mark's footsteps.