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Multicentric carpotarsal osteolysis syndrome (MCTO), an autosomal dominant disorder that often presents sporadically, features carpal–tarsal lysis frequently followed by nephropathy and renal failure. In 2012, mutations in the single‐exon gene MAFB were reported in 13 probands with MCTO. MAFB is a negative regulator of RANKL‐mediated osteoclastogenesis. We studied nine MCTO patients (seven sporadic patients and one affected mother and son) for MAFB mutation. We PCR‐amplified and selectively sequenced the MAFB region that contains the transactivation domain in this 323 amino acid protein, where mutations were previously reported for MCTO. We found five different heterozygous missense defects among eight probands: c.176C > T, p.Pro59Leu; c.185C > T, p.Thr62Ile; c.206C > T, p.Ser69Leu (four had this defect); c.209C > T, p.Ser70Leu; and c.211C > T, p.Pro71Ser. All 5 mutations …