Genomic DNA was extracted from peripheral-blood leukocytes. Whole-exome sequencing was performed for selected patients (at the Broad Institute), as previously described23 (see the Supplementary Appendix for details). We confirmed the identification of variants in the coding region of MKRN3 with the use of polymerase-chain-reaction (PCR) amplification followed by sequencing of the products with the use of the conventional Sanger method. For comparisons of the prevalence of truncating variants in the families and the exome variant server (hosted by the National Heart, Lung, and Blood Institute), we used Fisher’s exact test to compare the number of frame-shift or nonsense mutation carriers, counting each family member in the exome variant server once and counting both parents in each family to account for mutation searches across multiple offspring. For the analysis of unique variants, we included only one person (or one pair of parents) per variant in the analysis.
