Compared to a standard reference genome (hg18) the twins share 1.6 million variants. Of these variants, 9500 are in protein coding regions and approximately half are predicted to result in changes at the protein level (4600; nonsynonymous). On the basis of population data on DNA variation, a subset of 77 variants are seen with a population frequency appropriate for a rare genetic disease. On the basis of extensive review of the literature and clinical correlation, 3 likely candidate genes were selected by the authors of the study. M, million; K, thousand. [Adapted from data presented by Bainbridge et al. (15).]