The targeting vector was designed to alter exon 21 by changing the DNA sequence of codon 1117 from CGG-to-TGA (arginine-to-stop codon) resulting in a truncated 122 kDa protein. A loxP-flanked PGK-neo-pA cassette was also inserted downstream of the mutant stop codon. Cre-mediated recombination removed the floxed neo cassette. This non-sense mutation was first identified in three brothers diagnosed with schizophrenia/schizoaffective disorder between ages 16 and 21 (also with mild-to-moderate mental retardation) without showing obvious autistic features during their childhood.