Rh(mod) syndrome is a rare genetic disorder thought to result from mutations at a 'modifier' but not at the suppressor underlying the regulator type of Rh(null) disease. We studied this disorder in a Jewish family with a consanguineous background and analyzed RH and RHAG, the two loci that control Rh-antigen expression and Rh-complex assembly. Despite the presence of a d (D-negative) haplotype, no other gross alteration was found at RH, and cDNA sequencing showed a normal structure for D, Ce, and ce Rh transcripts in family members. However, analysis of RHAG transcript, which encodes Rh50 glycoprotein, identified a single G→T transversion in the initiation codon, causing a missense amino acid change (ATG[Met]→ATT[Ile]). This point mutation also occurred in the genomic region spanning exon 1 of RHAG, and its genotypic status in the mother and two children was confirmed by analysis of single-strand conformation polymorphism. Although blood typing showed a very weak expression of Rh antigens, immunoblotting barely detected the Rh proteins in the Rh(mod) membrane. In vitro transcription-coupled translation assays showed that the initiator mutants of Rh(mod) - but not those of the wild type - could be translated from ATG codons downstream. Our findings point to incomplete penetrance of the Rh(mod) mutation, in the form of 'leaky' translation, leading to some posttranslational defects affecting the structure, interaction, and processing of Rh50 glycoprotein.