<P> Originally there were four brothers (although Malcolm's oldest brother attended a military school away from home, so Malcolm was still the middle sibling left at home). A fifth son was introduced in the show's fourth season, a boy named Jamie . The boys are, from eldest to youngest: Francis, Reese, Malcolm, Dewey, and Jamie . In the final episode, Lois discovered she was pregnant with a sixth child . In the third season, Francis travels home (to celebrate his father's birthday) with an Alaskan girl named Piama, and reveals that they are married . </P> <P> During the first season, the writers decided to keep the family's last name a mystery . In the fifth season episode "Reese Joins the Army (1)", Reese uses a fake ID by the name of "Jetson" to lie about his age . In the series finale, "Graduation", Francis' employee ID reads "Nolastname" (or "No Last Name", a joke referring to the fact that the family name was never spoken aloud). In the same episode when Malcolm was introduced to give the graduation speech, the speaker announces Malcolm's name, but microphone feedback makes his surname inaudible, even though he does appear to mouth the phrase "No last name". </P> <P> In the pilot episode, the name Wilkerson can be seen on the name badge of the uniform Francis is wearing at Marlin Academy . In the sleeve notes for the season 5 DVD released in Europe, the family is referred to as the Wilkersons in the description for the episode "Block Party". British television advertisements and some Fox promotional material also occasionally referred to the family as the Wilkersons, although this name was never spoken aloud in the series . In an early version of the pilot, Malcolm had an exchange of dialogue with a classmate, who claims to have overheard his parents talking about Malcolm's family and asks him what his last name is . When Malcolm responds, "Wilkerson, why?" the friend replied with the punchline, "Oh...who are the' Pariahs'?" </P> <P> Out of the show's six leads, Cranston, Muniz, Berfield and Sullivan were the only ones to appear in all 151 episodes . </P>

Malcolm in the middle what's their last name