<P> Commissioner Francis Xavier Reagan is a fictional character and the protagonist of the CBS TV series Blue Bloods . He is portrayed by former Magnum P.I. star Tom Selleck . </P> <P> Frank is the younger son of Henry (Len Cariou) and Betty Reagan, born in Bay Ridge, Brooklyn, New York in the early 1950s . His older brother, Peter Christopher Reagan, died of leukemia at the age of 18 months over a year before Frank was born . </P> <P> Frank married Mary Margaret Reagan in the early 1970s, and they have four children together: Danny, Erin, Joe, and Jamie . Each of Frank's sons followed him into the NYPD: Danny (Donnie Wahlberg) is a Detective 1st Grade and lead investigator with the 54th Precinct's Detective Squad; Joe was a Detective with the Warrants Squad who was murdered on the orders of his partner Sonny Malevsky (Michael T. Weiss), the leader of a corrupt gang of police officers called the Blue Templar; and Jamie (Will Estes) is a patrol officer assigned to the 12th Precinct with a law degree from Harvard University . Frank's only daughter, Erin (Bridget Moynahan), is also in law enforcement as a Manhattan Assistant District Attorney . Erin was promoted to Deputy Bureau Chief of the NYCDA's Trial Bureau in 2013 . </P> <P> In addition to his children, Frank is also a loving grandfather to Erin's daughter, Nicky Reagan - Boyle (Sami Gayle), and Danny's two children, Jack (Tony Terraciano) and Sean Reagan (Andrew Terraciano). In one episode, it is mentioned that Frank's wife's grandfather was one of the builders on the Brooklyn Bridge in the 1870s and' 80s . Frank's dad's father, Charles Reagan, was an Irish immigrant who served in World War I before becoming a police officer . Charles was the one who taught him how to fish . In "Whistle Blower" (season 2, episode 10), Frank mentions his great - uncle Teddy Reagan, a member of the NYPD Mounted Unit, who spent a cold night in a boxcar with his horse, Dolly, and woke up in Pittsburgh . </P>

What happened to the third brother on blue bloods