CD45 is a receptor-type protein tyrosine phosphatase involved in the regulation of lymphocyte activation. Different CD45 isoforms are generated by alternative splicing of three variable exons (A, B and C). The pattern of CD45 splicing depends upon cell type and state of activation. CD45RA isoforms (containing exon A-encoded sequences) can usually be found on a subset of resting T cells, but not on activated T cells. We have recently described a variant pattern of CD45RA expression which is characterized by continuous expression of CD45RA molecules on activated and memory T cells. Here, we demonstrate that this phenotype is associated with heterozygosity for a point mutation at nucleotide position 77 of exon A, leading to a C → G transition. This mutation does not change the protein sequence of the CD45RA isoform. We conclude that position 77 is part of a motif necessary for splicing of exon A, which supports the hypothesis that sequences within exons have significant effects on alternative splicing. The mutation of this motif might prevent binding of a transacting splice factor. In the heterozygous state, this mutation is not associated with impaired T cell reactivity. Functional consequences of the homozygous state remain to be elucidated.