A targeting construct was designed to insert a loxP-flanked neomycin (neo) resistance cassette downstream of exon 3 of the methyl CpG binding protein 2 (Mecp2) gene. A point mutation was introduced in exon 4, corresponding to human amino acid 255, resulting in a non-sense mutation, R255X, commonly found in humans carrying Rett Syndrome (RTT). Cre mediated recombination removed the floxed selection cassette. Western blot analysis indicates absence of full-length protein or the predicted truncation product.