These symbols represent the four basic chemical letters, or bases, the body uses to form DNA--guanine, cytosine, adenine and thymine.

FORBES: Magazine Article

We drove through the city when, as you all remember, the symbols on the door -- the circle with the X and the four numbers that denoted how people were here, how many people were lost, whether or not -- making sure everybody was accounted for.

WHITEHOUSE: 10/14/09: White House Press Briefing

Their numbers grew from four to about 20, making them the largest herd of hippos outside of Africa and famous symbols of the cocaine industry.

BBC: Inside Pablo Escobar��s Medell��n cartel