Representation of the exons and introns in A2 and O1valleles and the new hybrid allele. Exons are represented as thick boxes and introns as thin boxes. They are drawn approximately to scale except for the approx 13 kbp long intron 1. Filled regions indicate an A2 sequence and unfilled regions an O1vsequence. The hatched region in the hybrid allele indicates the cross-over region. The grey area is presumed to have an O1vallele sequence, but lack of known mutations in exons 1 and 2 and the unsequenced intron 1 does not allow proof of this. The asterisks indicate the localisation of the four missense mutations compared to the common A2 allele.