Humans with OKT4-epitope deficiency have a single nucleotide base change in the CD4 gene, resulting in substitution of TRP240 for ARG240.

The OKT4 epitope of the CD4 cell-surface protein has been shown to be polymorphic in white, black, and Japanese populations. The variable phenotypic expression is due to an alteration of the OKT4 epitope, since those persons lacking reactivity with OKT4 monoclonal antibody (mAb) are reactive with OKT4A-F mAb as well as other mAb specific for CD4. To determine the nature of this polymorphism at the gene level, we sequenced polymerase chain reaction-amplified genomic DNA containing the CD4-V3 and -V4 exons from American black subjects who are OKT4-normal, OKT4-negative heterozygous, or OKT4-negative homozygous. Comparison of the sequences revealed that the two CD4 exons are identical except for a cytosine-to-thymidine transition occurring at nucleotide position 868. This alters the first codon position of mino acid 240 and results in a tryptophan residue replacing an arginine residue. The change was also found in white and Japanese persons who are OKT4-negative.