Mutations in the ARX gene can result in many different phenotypes, including phenotypes associated with severe brain malformations and less severe phenotypes associated with syndromic or non-syndromic forms of XLMR. There seems to be a consistent genotype-phenotype correlation and both interfamilial and intrafamilial variability of expression of some of the mutations, particularly the common 428-451dup(24 bp) mutation. Familiarity with the phenotypic spectrum of ARX mutations is helpful in determining when to request ARX mutation analysis.