Deuteranomaly, and to a lesser extent deuteranopia, are associated with a 5' green-red 3' (G-R) visual pigment hybrid gene. Such G-R hybrid genes may also exist in males with normal color vision. This discrepancy can be explained if the position of the G-R hybrid gene in the at-ray determines the probability of ifs expression in the retina. In accord with our previous findings, we confirm that when a G-R hybrid gene occupies the second position in the array, a deutan defect results (N = 2); whereas when a normal green pigment gene does, normal color vision ensues (N 3). Regarding gene expression as a function of position in the army, in 5 postmortem retinae from individuals whose arrays contained one red and two green pigment genes that differed at a polymorphic site in exon 5, only the green pigment gene that occupied the second position in the array was expressed into retinal mRNA. In conclusion, our data suggest that only the first two genes in the X-chromosome linked visual gene army are expressed in the retina and, therefore, influence the color vision phenotype. Green-red hybrid genes cause deuteranomaly only if they occupy the second position in the array. (C) 2000 John Wiley & Sons, Inc. Col Res Appl, 26, S79-S83, 2001.