A relative handful of genes accounts for the inherited factors shaping a person's appearance. Humans have an estimated 19,000–20,000 human protein-coding genes. Richard Sturm and David Duffy describe 11 genes that affect skin pigmentation and explain most variations in human skin color, the most significant of which are MC1R, ASIP, OCA2, and TYR. There is evidence that as many as 16 different genes could be responsible for eye color in humans; however, the main two genes associated with eye color variation are "OCA2" and "HERC2", and both are localized in chromosome 15.