In 9 of 22 couples (41%), no informative SNPs were found in the 5′ region, main region, or 3′ region (two cases each with Charcot–Marie–Tooth disease (CMT) and myotonic dystrophy 1 (DM1) and one case each with von Hippel–Lindau syndrome, Duchenne muscular dystrophy, Marfan syndrome, spinocerebellar ataxia, and ornithine transcarbamylase deficiency. The gene discussed is OTC; the disease is Duchenne muscular dystrophy.