Marshall Warren Nirenberg (1927- present) shared the Nobel Prize in Physiology or Medicine in 1968 with Har Gobind Khorana and Robert W. Holley for describing the genetic code and how it helps make protein. Using RNA composed solely of uracil, Nirenberg showed that the genetic code for phenylalanine was UUU (three uracil bases in a row). This was the first step in deciphering the codons and the first demonstration of messenger RNA. Within a few years, Nirenberg’s research team found that three repeats of adenosine (AAA) produced the amino acid lysine, three cytosine repeats (CCC) produced proline, and three guanine repeats (GGG) produced nothing at all. The three scientists eventually developed a method for determining the genetic code from pieces of tRNA, which greatly sped up the assignment of codons. Fifty codons were identified by Nirenberg this way.