<P> Mark first appears in season two, introduced as a highly respected otolaryngologist sub-specialized in plastic surgery and the childhood best friend of neurosurgeon Derek Shepherd (Patrick Dempsey). In his first appearance, he flirts with Meredith Grey, and Derek punches him in the face . Derek explains that Mark had an affair with his wife, Addison (Kate Walsh) while they were living in New York . Mark travels to Seattle, intent on convincing Addison to return with him, but his offer is rejected and Derek declines to renew their friendship . Mark returns during season three at Addison's drunken behest, but she again rejects him once sober . Undeterred, Mark sells his successful private practice (which he previously shared with Derek) and takes over the plastics program at Seattle Grace Hospital . During Meredith's morphine rampage, Mark finds out about his nickname McSteamy which was given to him by her during his first trip to Seattle back when he attempted to get Addison back and earn Derek's friendship back . It is later revealed that Mark has at some point slept with all of Derek's sisters . Mark has a brief fling with Addison's friend, orthopedic surgeon Callie Torres (Sara Ramirez), and develops a friendship with Derek's girlfriend, intern Meredith Grey (Ellen Pompeo). It is revealed that after Derek left New York, Mark and Addison continued their relationship for two months, during which she conceived and aborted his child . Just weeks after moving to Seattle he quickly observes that Derek's true love was Meredith and tries to convince Addison that her marriage with Derek was over . Mark enters into a sixty - day abstinence pact with Addison, agreeing that if they can remain celibate for that time, Addison will give their relationship another chance . Addison ultimately breaks the pact by having sex with intern Alex Karev (Justin Chambers), and soon thereafter departs from Seattle to work in Los Angeles . </P> <P> Mark supports Derek when Meredith comes close to dying after drowning, and the two are able to rekindle their friendship . It is eventually revealed that he and Derek grew up together and that he, having lost his mother as a child and emotionally neglected by his father, considered Derek's mother as his maternal figure . As he was an only child and Derek was the only son, they became close friends and he was Derek's best man at the latter's wedding to Addison . He also forms a friendship with Callie, and develops an attraction to cardiothoracic surgeon Erica Hahn (Brooke Smith). Erica does not reciprocate his feelings, and he is supportive of Callie when she and Erica have a brief relationship . When Addison shows up at Seattle Grace for a case, Mark attempts to sleep with Addison again but she refuses . The nurses later start a club against him called Nurses United Against Mark Sloan . After a string of unresolved sexual tension on his part, Mark then finds himself seduced by Lexie Grey (Chyler Leigh), despite Derek's request that he keep their relationship platonic even before their relationship had begun . They begin a sexual relationship, which Lexie halts until he is willing to publicly admit to being her partner . Mark eventually agrees and tells Derek, who attacks him . Animosity exists between them for several episodes, before they once more repair their friendship . Mark and Lexie's relationship becomes increasingly serious . He meets her father, Thatcher (Jeff Perry); and at the end of season five, asks her to move in with him . Though Lexie initially declines, he remains committed to their relationship, and she agrees early in season six . </P> <P> Mark discovers that he has an 18 - year - old daughter, Sloan Riley (Leven Rambin), who arrives seeking his support after becoming pregnant and being evicted by her mother . Mark permits her to move in with him, which angers Lexie . When Lexie makes it clear that she knew that Mark would choose his own daughter over her, she puts an end to their relationship . Sloan experiences difficulties in her pregnancy, so Mark takes her to LA to be treated . He and Addison once again sleep together, but when he comes back he tells Lexie because he doesn't want to lie to her and wants them to get back together, but when she tells him she slept with Alex it puts an end to their relationship . When Sloan decides to give her baby up for adoption, Mark and Callie offer to raise it together . Sloan declines his offer and leaves Seattle, leaving Mark distraught . He enters into a relationship with cardiothoracic surgeon Teddy Altman (Kim Raver), despite the fact she is in love with their trauma colleague, Owen Hunt (Kevin McKidd). Sloan returns unexpectedly and gives birth to a son . She reconsiders having the baby adopted, but when Mark reassures her that he will support her no matter what, gives the baby to an adoptive couple from Washington . </P> <P> In the sixth - season finale, a gunman commits mass murder at the hospital . Mark helps Lexie save Alex, her new boyfriend . In the aftermath of the shooting, Lexie has a nervous breakdown and Mark has her committed to the psychiatric unit . Their friendship is strained, however Lexie softens towards him when she learns that he is still in love with her . As Lexie goes to talk to Mark about him still being in love with her, she sees Mark entering his apartment with Derek's sister Amelia kissing her and ultimately having sex with her . When Callie's girlfriend, pediatric surgeon Arizona Robbins (Jessica Capshaw), breaks up with her, she and Mark have a drunken one - night stand . Shortly thereafter, he reconciles with Lexie, but is delighted when Callie discovers she is pregnant with his child . Lexie, however, is dismayed by the news and leaves Mark . </P>

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