been extensively studied as a stand-in for human ailments. In humans, two genes for colour vision are found on the X chromosome. Typically, one gene (OPN1LW) produces a pigment that is most sensitive to the 564 nm wavelength, while the other gene (OPN1MW) produces a pigment most sensitive to 534 nm. In squirrel monkeys, there is only one gene on the X chromosome but it exists in three varieties: one is most sensitive to 538 nm, one to 551 nm, and one to 561 nm. Since males have only one X chromosome, they are dichromatic, although with different sensitivities. Females have two X chromosomes, so some of them can have copies of two different alleles. The three alleles seem to be equally common, leading to one-third of females being dichromatic, while two-thirds are trichromatic. Recently, gene therapy has given the human OPN1LW gene to adult male squirrel monkeys, producing behaviour consistent with trichromatic colour vision. Gallery References External links Primate Info Net Saimiri Factsheet Reforestation for the endemic squirrel monkey of Panama and Costa Rica View the squirrel monkey genome in Ensembl