Certain stretches of amino acids can fold independently around a zinc ion to form modules called zinc fingers (bracketed regions at top). A major clue (bottom) to the folding pattern came from inspection of the sequence of amino acids in the protein TFIIIA (now known as a C2H2 zif protein). The bulk of the protein can be arranged into nine successive sections, or sequence units (numbered), that exhibit important similarities: they include, at virtually identical positions, a pair of cysteine amino acids (gold C's) a pair of histidine amino acids (red H's) and (with the possible exception of section 7) three hydrophobic amino acids (green letters). (Asterisks mark unimportant breaks in the pattern). These observations, added to the biochemical findings, led to the proposal that the cysteine and histidine pairs in every module would combine with a single zinc ion (large yellow spheres in top image), causing the amino acids between those pairs to loop out as shown. At the same time, the three hydrophobic amino acids would stablize the arrangement.