Supplementary Material for: EX-HOM (EXome HOMozygosity): A Proof of Principle

<i>Objective:</i> We provide the proof of principle that exome sequencing of only two affected siblings born to first-cousin parents is capable of directly identifying a single candidate gene for an autosomal recessive disorder. This strategy, which we call EX-HOM (EXome HOMozygosity), combines in a single step the capacity of exome sequencing to identify all the coding variants present in a genome with the property of homozygosity mapping to limit the search for candidate genes to specific chromosomal regions. <i>Methods:</i> We sequenced the exomes of two siblings born to first-cousin parents affected with dysmyelinating leukodystrophy and spastic paraparesis caused by a mutation in <i>FA2H</i>. We used exome sequencing data to identify homozygous regions shared by the two affected siblings (EX-HOM regions), compared them with the regions of maximum LOD score obtained with SNP genotyping, and selected the candidate variants within. <i>Results:</i> We identified regions of shared homozygosity (&gt;1 Mb) accounting for about 290 Mb, containing only 3 candidate variants. Among these, the <i>FA2H</i> mutation remained the only plausible one. <i>Conclusion:</i> In single consanguineous pedigrees with a few affected sibs, EX-HOM can be a one-step approach to identify the candidate genetic defect, bypassing obstacles such as genetic heterogeneity and the need for large pedigrees.