The IL-1β knockout was developed by Zheng et al. in the laboratory of Lex Van der Ploeg at Merck Research Laboratories. AB 2.1 (129S7/SvEvBrd) embryonic stem cells, gene targeted by positive-negative selection, were injected into C57BL/6 blastocysts. The resultant chimeras were backcrossed for three generations to the MHC congenic B10.RIII-H2^r to transfer the H2^r haplotype to the targeted mutant. The IL-1ß heterozygotes were intercrossed to achieve homozygosity. This model was distributed as Taconic line 612, which is no longer available. The line was subsequently backcrossed by a third party to C57BL/6J and the H2r haplotype was removed. This current version of the model was imported to Taconic in 2018 and is congenic on C57BL/6J (NE7) with some residual genomic DNA from the 129 ES cells according to SNP testing.