For example, a recent study in a large cohort of unresolved families with XLID revealed that 20% of families carried pathogenic variants in established XLID genes (Hu et al., 2015), as well as revealing seven novel XLID genes (CLCN4, CNKSR2, FRMPD4, KLHL15, LAS1L, RLIM, and USP27X) and two candidates (CDK16 and TAF1). This evidence concerns the gene USP27X and cask-related x-linked intellectual disability.