Multicentric carpotarsal osteolysis (MCTO) is an autosomal dominant disease of the skeleton characterised by progressive destruction of carpal and tarsal bones. Recently, it has been demonstrated that this disease is caused by heterozygous mutations in the gene for the transcriptional repressor MAFB. We analysed genomic DNA and RNA from leucocytes of a female patient diagnosed with MCTO. We identified the mutation c.161C>T in the genomic sequence and in the expressed messenger RNA for MAFB. This is the second report of the c.161C>T mutation in a MCTO patient. Since the parents do not possess this mutation, the daughter must have acquired a de novo mutation. At the level of the gene, this mutation is found at a CpG dinucleotide sequence, suggesting that DNA methylation was involved in the occurrence of the DNA aberration. At the level of the protein, the mutation exchanges a serine with a leucine residue at a position on MAFB that can become phosphorylated in the wild-type protein. MAFB negatively regulates the RANKL-dependent differentiation of monocytes into osteoclasts. It is likely that the mutation will affect the phosphorylation status of the protein and its biological activity. When the activity of the transcriptional repressor is reduced, osteoclastogenesis will be increased, which might explain the carpotarsal bone destruction observed in the patient.