Genome-wide genotyping data were generated with the OmniExpress platform (∼500,000 markers) and used for performing homozygosity mapping. Tracts of potential homozygosity greater than 1 Mb were initially identified for each individual. The boundaries of overlapping homozygosity tracts shared by all three affected individuals were subsequently determined. Coverage across regions of shared homozygosity was calculated with BedTools against the Consensus Coding Sequence (CCDS) definition of the exome and was expressed as the percentage of target bases covered by at least one read (see Table 1). Mean coverage of CCDS genes across all homozygous regions was 90.7%.