Methods and kits for the diagnosis of illnesses related to protocadherin 19 (PCDH 19) protein deficiency or altered PCDH 19 protein function, in particular EFMR (Epilepsy and Mental Retardation limited to Females) are provided, as well as methods and kits for the identification of a predisposition to such illnesses and methods of screening subjects to identify carriers of such illnesses and methods and kits for the therapeutic or prophylactic treatment of PCDH 19 deficiency or altered PCDH 19 protein function. Further, nucleotide and amino acid sequences corresponding to a complete PCDH19 open reading frame (ORF), mutant sequences encoding non-functional PCDH19 mRNA or altered PCDH19 mRN A are described along with transformed cells and non-human transgenic animals comprising wild-type or mutant PCDH19 ORF nucleotide sequences.
