Probable inactive allele of Nlrp1b, which lacks a CARD domain, suggesting that it is not able to form an inflammasome (PubMed:16429160). Contrary to Nlrp1b allele 1, allele 4 is not activated by B.anthracis lethal toxin and no other activation signal is reported (PubMed:16429160). Expressed in macrophages. Contrary to Nlrp1b alleles 1, 2, 3 and 5, allele 4 is missing a CARD domain, which has been shown in other alleles to be involved in the interaction with CASP1 and CASP4/CASP11. It is thus unclear if this allele is able to promote inflammasome assembly. The FIIND (domain with function to find) region may be involved in homomerization, but not in CASP1-binding (PubMed:16429160). In allele 4, the FIIND region is truncated and hence does not contain the activating cleavage reported for allele 1 (PubMed:16429160). Consequently, it may not undergo autocatalytic cleavage in this region (PubMed:16429160). Nlrp1b gene is extremely polymorphic. 5 alleles have been described in 18 inbred strains: 1 (AC Q2LKW6), 2 (AC A1Z198), 3 (AC Q2LKV5), 4 (this entry) and 5 (AC Q0GKD5). These alleles define susceptibility to B.anthracis lethal toxin (LT). Alleles 2 (carried by A/J, C57BL/6J and I/LnJ), 3 (AKR/J, NOD/LtJ and SJL/J) or 4 (DBA/2J, P/J and SM/J) are not activated by LT. Alleles 1 (carried by 129S1/SvImJ, BALB/cJ, C3H/HeJ, CBA/J, FVB/NJ, NON/ShiLtJ, NZO (NZO/HlLtJ) and SWR/J strains) and 5 (CAST/EiJ) confer macrophage susceptibility to LT. In susceptible strains, infection by Bacillus anthracis leads to IL1B release, neutrophil recruitment and macrophage pyroptosis. This early inflammatory response confers increased resistance to infection (PubMed:16429160). The sequence shown in this entry is that of allele 4 (PubMed:16429160). Three tandem Nrlp1 paralogs, Nrlp1a, Nrlp1b and Nrlp1c, have been identified. Nlrp1c is predicted to be a pseudogene. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. Belongs to the NLRP family.