Played by Mark-Paul Gosselaar: A confident youngster who spent the last few years in Narcotics, Clark received the NYPD equivalent of a battlefield commission, earning his gold detective shield after heroic action during an undercover operation gone awry. When offered his choice of commands, he picked the 15th Precinct. That decision drove a wedge between John and his father, John Clark Sr., a by-the-book veteran who didn't want his son spending time in that "hellhole" -- or learning how to be a thug like Sipowicz, whom Sr. loathed from past experience. Despite some early screw-ups, John's on-the-job training has been surprisingly quick; he's now good enough in the interview room that the "Junior" nickname Andy slapped on him doesn't sound condescending, but affectionate. However, with that growth has come tragedy. John's dad committed suicide after an IAB scandal, and so did John's bipolar ex-girlfriend, Dr. Jennifer Devlin.

Mark-Paul Gosselaar

Born on March 1, 1974, Mark-Paul Gosselaar is the second former child star in a row to find an adult home as an "NYPD Blue" cop.

Born in Panorama City, California, the youngest of four siblings (and the only one not born in the Netherlands), the young Mark-Paul was blonde enough, cute enough and talented enough to get guest starring roles on shows like "Highway to Heaven" and "Punky Brewster."

In 1988, he was cast in a regular role as troublemaking student Zack Morris on Hayley Mills' Disney Channel sitcom "Good Morning, Miss Bliss." After 13 episodes, the show moved to NBC's Saturday morning in 1989 -- without Miss Bliss. Under the new title "Saved by the Bell," the focus shifted to Zack and the other kids. The show ran for four years, became wildly popular among adolescents and launched a number of acting careers (including Elizabeth Berkley, who guest-starred on "Blue" a few years ago as a reporter who slept with Baldwin).

Zack and friends graduated to primetime for a short-lived series called "Saved by the Bell: The College Years." After bouncing around in low-budget TV movies with names like "She Cried No" and "Dying to Belong," he got his first real adult roles on two WB drama series: "Hyperion Bay" and "D.C.," neither of which stuck around long.

Fortunately for Mark-Paul, "NYPD Blue" producer Steven Bochco had never heard of "Saved by the Bell," much less seen it. The two initially met when Mark-Paul auditioned for a role on "Philly"; the part went to Tom Everett Scott, but Bochco was impressed enough to want to stay in the Mark-Paul Gosselaar business. He hired Mark-Paul to an "NYPD" contract, several months before Rick Schroder officially decided to leave the show. (Bochco says he would have had no problem keeping both actors had Schroder decided to stay.)

Mark-Paul married actress Lisa Ann Russell, whom he met on the set of "Saved by the Bell: The College Years," in August of 1996.

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