Also used as the serial number of one of the stormtroopers killed (or perhaps only knocked out) by Luke, Han, and Obiwan in the book version of Star Wars (known in the movie as TK421) written by George Lucas himself. And I quote

 But the absence of the two troopers was noticed, soon thereafter. A gantry officer passing the window of a small command office near the freighter entrance glanced out, frowning when he saw no sign of the guards. Concerned but not alarmed, he moved to a comlink and spoke into it as he continued to stare at the ship.
 "THX-1138, (emphasis mine) why aren't you at your post? THX-1138, do you copy?"
 The speaker gave back only static.
 "THX-1138, why don't you reply?" The officer was beginning to panic when an armored figure descended the ramp and waved toward him. Pointing to the portion of his helmet covering his right ear, the figure tapped it to indicate the comlink inside wasn't working.
 Shaking his head in disgust, the gantry officer gave his busy aide an annoyed look as he made for the door. "Take over here. We've got another bad transmitter. I'm going to see what I can do." He activated the door, took a step forward as it slid aside-- and stumbled backward in a state of shock.

Sorry if that was longer than needed, but I like the "what happens next?" people who haven't seen Star Wars are now thinking. I urge you to go out and see it, or read the book. If you want the ISBN please see my writeup in Star Wars.