We report here the pre-mRNA splicing of the human glycophorin Mz (HGpMz) gene altered by a defective donor splice site introduced into the third intron via the mechanism of gene conversion. We found that the directional transfer from HGpB(delta) to HGpA(alpha) of a 145-base pair silent homologous segment had resulted in HGpMz as an alpha-beta-alpha hybrid gene with a G-T transversion in the consensus GT motif of the 5'-donor splice site. Transcript analysis showed the presence in the Mz reticulocytes of three major glycophorin cDNA species of which two occurred as the shortened and aberrantly spliced products of HGpMz pre-mRNA lacking the sequences encoded by one and two exons, respectively. The skipping of exon III results in one HGpMz protein with 99 amino acids that bears the M and Sta blood group antigens, whereas the coincident skipping of both exon III and exon IV leads to the expression of a minor HGpMz protein with 86 amino acids. These results indicate that the translocated defective donor splice site not only causes uniform skipping of the upstream exon but also affects the processing of the downstream exon. Complementing our previous studies, the finding that the active and inactive splice sites can be transposed by DNA recombination and alternatively used to construct new glycophorin alleles reveals a novel mechanism for sequence diversification in human genes.