AbstractThe three basic DNA‐binding domain mutations of the microphthalmia‐associated transcription factor (Mitf), Mitfmi/mi, Mitfor/or, and Mitfwh/wh affect osteoclast differentiation with variable penetrance while completely impairing melanocyte development. Mitfor/or mice exhibit osteopetrosis that improves with age and their osteoclasts form functional multinuclear osteoclasts, raising the question as to why the Mitfor/or mutation results in osteopetrosis. Here we show that Mitfor/or osteoclasts express normal levels of acid phosphatase 5 (Acp5) mRNA and significantly lower levels of Cathepsin K (Ctsk) mRNA during receptor activator of nuclear factor kappa B (NFκB) ligand (RANKL)‐mediated differentiation. Studies using chromatin immunoprecipitation (ChIP) analysis indicate that low levels of Mitfor/or protein are recruited to the Ctsk promoter. However, enrichment of Mitf‐transcriptional co‐activators PU.1 and Brahma‐related gene 1 (Brg1) are severely impaired at the Ctsk promoter of Mitfor/or osteoclast precursors, indicating that defective recruitment of co‐activators by the mutant Mitfor/or results in impaired Ctsk expression in osteoclasts. Cathepsin K may thus represent a unique class of Mitf‐regulated osteoclast‐specific genes that are important for osteoclast function. J. Cell. Physiol. 220: 230–237, 2009. © 2009 Wiley‐Liss, Inc.