Immunostaining of p57KIP2, the product of the paternally imprinted gene, CDKN1C, shows positive staining of the cytotrophoblast and villous stroma in all pregnancies except for CHM, in which both sets of chromosomes are paternally derived [5], and is thus a useful discriminator in the diagnosis of CHM [3, 5, 10]. This evidence concerns the gene CDKN1C and choroideremia.