Objective. To gain insight into the molecular bases of genetically unexplained periodic fever syndromes (PFS) by screening NLRP12, a gene in which only a nonsense and a splice site mutation have so far been identified, and to assess the functional consequences of the identified missense variation.Methods. NLRP12 was screened for mutations by direct sequencing. Functional assays were performed in HEK 293T cells stably expressing the proapoptotic protein ASC and procaspase 1, in order to determine the effects of normal and mutated NLRP12 proteins on speck formation, caspase 1 signaling, and NF-B activation.Results. A heterozygous NLRP12 missense mutation involving a CpG site (c.1054C>T; p.Arg352Cys) was identified in exon 3, which encodes the nucleotidebinding site (NBS) of the protein, in 2 patients from different countries and carrying different NLRP12 haplotypes. The mutation, which does not alter the inhibitory effect of NLRP12 on NF-B activation, increases speck formation and activates caspase 1 signaling. To define this new class of PFS, we propose the term NLRP12-associated disorders (NLRP12AD).Conclusion. Given the rarity of known NLRP12-associated disorders, the identification of this NLRP12 molecular defect contributes to the delineation of the clinical spectrum associated with mutations in this gene and highlights the importance of screening NLRP12 in patients presenting with unexplained PFS. This study also demonstrates, by means of functional assays, the deleterious effect of this recurrent missense mutation; the gain of function for speck formation and caspase 1 signaling associated with this NBS mutation is consistent with the inflammatory phenotype of PFS.Periodic fever syndromes (PFS) are Mendelian autoinflammatory disorders characterized by recurrent episodes of fever and systemic inflammation, sometimes complicated by amyloidosis. Recently, the first two recognized disease-causing mutations (a nonsense mutation and a splice site mutation) have been identified in the NLRP12 gene (also known as NALP12 or MONARCH-1) in patients with a clinical picture suggestive of a cryopyrin-associated periodic syndrome (CAPS) (1). CAPS represent a subgroup of PFS comprising the familial cold-induced autoinflammatory syndrome, the Muckle-Wells syndrome, and the chronic infantile neurologic, cutaneous, articular (CINCA) syndrome, also known as neonatal-onset multisystem inflammatory disease. These three disorders have been associated with mutations in NLRP3, which, like NLRP12, is a member of the NLRP family.Diagnosis of PFS remains difficult, for the following reasons: there is no pathognomonic sign of these disorders; many patients present with symptoms that do not fit any classification; the majority of patients referred today are sporadic cases, precluding intrafamilial segregation analyses; and the majority of mutations identified in PFS genes correspond to missense variations, whose deleterious effects are often questionable.