C57BL/6 mice deficient in IL15 were generated by gene targeting in the laboratory of Dr. Jacques Peschon of the Immunex Corporation in 1996. The IL15 gene was disrupted by replacing a 7.5 kb fragment containing IL15 exons 3-5 with a PGK-neo cassette. The disrupted gene was introduced into ES cells of C57BL/6 origin developed at Immunex and injected into BALB/c blastocysts to create chimeric mice. Resulting male chimeric animals were bred to C57BL/6 females and offspring heterozygous for the IL15 knockout allele were crossed to generate IL15 deficient mice (IL15-/-). Different C57BL/6 sublines were used during the development of this line and therefore an exact control mouse was not available. However, C57BL/6NTac mice were used as controls in the original studies. Recently we completed backcrossing these mice an additional 5 times onto C57BL/6NTac mice and intracrossed them to make them homozygous for the IL15 knockout allele. We now consider the Taconic B6 (C57BL/6NTac) mouse to be a more appropriate control for line 4269.