A single nonrecurrent truncating variant was found in 19 genes from the X-specific part of the X chromosome. Analysis of segregation in each family revealed that the truncating variant in nine genes (ATXN3L, DRP2, MAP3K15, MAP7D3, RPL9P7, SATL1, SSX6, SYTL5 and ZCCHC13) did not segregate with mental retardation (Table 3 and Supplementary Fig. 1). In nine of the remaining ten genes there was full segregation with the disease and in one, VSIG4, additional DNA samples were unavailable for testing. A heterogeneity lod score of 2.4 was obtained for the truncating variants in these 19 families, with mental retardation in 43% attributable to the truncating variant. Sequencing of the complete coding sequences of the 19 genes in male controls revealed one or more truncating variants in ATXN3L, BEX4, MAP3K15 and P2RY4 (Table 3). Furthermore, likely MR-causing abnormalities in MECP2, SLC9A6 and IL1RAPL1 have recently been found in affected individuals with single non-recurrent truncating variants in FAM47B, SATL1 and SAGE1, respectively (Table 3). Taken together, the results from 6 of the 19 genes with a single nonrecurrent truncating variant remain compatible with
