To identify the cause of disease in an adult patient presenting with recent‐onset fevers, chills, urticaria, fatigue, and profound myalgia, who was found to be negative for cryopyrin‐associated periodic syndrome (CAPS) NLRP3 mutations by conventional Sanger DNA sequencing.

We performed whole‐exome sequencing and targeted deep sequencing using DNA from the patient's whole blood to identify a possible NLRP3 somatic mutation. We then screened for this mutation in subcloned NLRP3 amplicons from fibroblasts, buccal cells, granulocytes, negatively selected monocytes, and T and B lymphocytes and further confirmed the somatic mutation by targeted sequencing of exon 3.

We identified a previously reported CAPS‐associated mutation, p.Tyr570Cys, with a mutant allele frequency of 15% based on exome data. Targeted sequencing and subcloning of NLRP3 amplicons confirmed the presence of the somatic mutation in whole blood at a ratio similar to the exome data. The mutant allele frequency was in the range of 13.3–16.8% in monocytes and 15.2–18% in granulocytes. Notably, this mutation was either absent or present at a very low frequency in B and T lymphocytes, in buccal cells, and in the patient's cultured fibroblasts.

Our findings indicate the possibility of myeloid‐restricted somatic mosaicism in the pathogenesis of CAPS, underscoring the emerging role of massively parallel sequencing in clinical diagnosis.