A splice mutation was introduced to the beginning of exon 3 along with an in-frame GFP-PGK-neo cassette. A threonine to isoleucine substitution at amino acid 68 (T68I), a mutation found in human X-linked lymphoproliferative disease (XLP) that inhibits protein binding, was also incorporated to ensure a null allele. Western blot analysis of thymocyte protein from homozygous mutant animals failed to detect a protein product. Antibodies against GFP also failed to detect a protein product.