The molecular basis of C7 deficiency has been investigated in two Irish families and a number of Israeli families of Moroccan Sephardic Jewish origin. Exon PCR and sequencing revealed a heterozygous point mutation at the 3' splice acceptor site of intron 1 in one Irish family. In the other Irish family, exons 7 and 8 failed to amplify and they were shown to be deleted. Marker haplotype studies of the C6 and C7 gene region and Southern blots show that the Irish family with the splice defect also segregate for the deletion, which is not easily detected in heterozygotes. The Israeli C7-deficient cases all share a C7 haplotype and are homozygous for a mis-sense mutation in exon 9. However, one individual is heterozygous for markers at adjacent C6 loci, showing that there has been an intergenic recombination and suggesting that the deficiency mutation is of appreciable antiquity.