Drs. Ji-Liang Gao and Philip Murphy in the Laboratory of Host Defenses, NIAID, NIH generated the Ccr1 Knockout mouse in 1997. The Ccr1 gene was disrupted by replacing 352 base pair fragment in the middle of the intronless open reading frame with a neomycin resistance gene. The Ccr1 gene was disrupted in 129S4/SvJae-derived J1 ES cells. Resultant chimeras were backcrossed to C57BL/6NAi mice for 10 generations in the NIAID barrier facility at Taconic. At this point, it was discovered that the C57NL/6NAi line had become contaminated with IFNg knockout gene. Line 4087 was then backrossed an additional 5 times to C57BL/6NTac in order to eliminate the contaminating gene and put the strain on the Taconic C57BL/6 subline.