A large Japanese family in which some members were homozygous or heterozygous for OKT4 epitope deficiency was studied. Homozygotes, heterozygotes, and normal individuals were identified by differences in the number of OKT4 epitopes on the surfaces of lymphocytes. This deficiency was transmitted as an autosomal codominant trait. The internalization of CD4 molecules and the production of IL-2 by lymphocytes of these subjects were examined. The OKT4 epitope was not needed for internalization of CD4 molecules, and IL-2 was produced in the same amounts by these different kinds of subjects. DNA from four clones lacking OKT4 established from four individuals of this family was sequenced. As reported elsewhere for different subjects, a single nucleotide substitution (CGG-->TGG) was found in all four cell lines. The mutation results in arginine being replaced by tryptophan. Analysis showed different hydrophobicity at positions 239 and 240 from the control, probably giving rise to a conformational change in CD4 accounting for lack of reactivity with the OKT4 monoclonal antibody. The incidence of homozygotes in the Japanese population was found to be 0.47% by examination of 1478 random samples, and on the basis of this value, the incidence of heterozygotes was estimated to be 12.8%.