Molecular genetic analysis for the B(3) allele.

Molecular genetic analysis of 14 samples from unrelated individuals with the B(3) phenotype is reported here. Two different molecular changes in the blood group B gene were observed. One case was demonstrated to possess a 247G --> T mutation, which predicts an Asp83Tyr alteration. The B genes of the other 13 cases were shown to have a G --> A mutation at the +5 nucleotide of intron 3 (intervening sequence 3 [IVS3] + 5G --> A). Reverse transcription polymerase chain reaction analysis showed that the complete exon 1-exon 7 B transcript was absent, and transcripts that skipped exon 3 were instead present in the RNA sample from the B(3) individual with the IVS3 + 5G --> A mutation. The result shows that the IVS3 + 5G --> A mutation destroys the conserved sequence of the splice donor site and leads to the skipping of exon 3 during messenger RNA processing. The B(3) transcript without exon 3 predicts a B-transferase product that lacks 19 amino acids in the N-terminal segment.