Two point mutations were introduced into exon 14 to substitute a premature stop codon for the aspartate at amino acid position 449 (D449X). This mimics the "Artemis-P70" allele in humans where a 7-nt deletion results in a frameshift at D451 followed by a premature stop codon 10 amino acids downstream. For targeting purposes, a loxP-flanked neomycin resistance cassette was included downstream of exon 14 that was subsequently removed by crossing with cre transgenic mice. Expression of the mutant allele was similar to the endogenous allele as determined by Northern and Q-PCR analysis of splenic RNA from homozygote mice.