We report the program of gene expression during osteoclast formation from RAW264.7 cell precursors in response to RANK-ligand (RANK-L) using a combination of quantitative real time PCR and Affymetrix gene chip assays. We found that genes obligatory to osteoclast formation and function, namely tartrate-resistant acid phosphatase, cathepsin K, beta3 integrin, and calcitonin receptors, were Up-regulated by RANK-L markedly by up to similar to2000-fold. In contrast, we found a cluster of genes that were significantly down-regulated: these included interleukin-18, insulin-like growth factor-1, interleukin-6 receptor, and cathepsins B, C, and L. These results from real time PCR were broadly concordant with those obtained from Affymetrix. We also explored the expression of the transcription factors of the NFAT and NFkappaB family at days 3 and 5 of culture. Whereas NFATcl expression was increased significantly at days 3 and 5 following RANK-L exposure, there were no significant increases in the expression of NFkappaB subunits, namely p65, p50, c-Rel. IkappaBalpha, and IkappaBbeta. There were also no significant differences in transcription modulator expression between days 3 and 5, except for c-Rel and NFATc4, which were both decreased significantly at day 5. The studies suggest RANK-L regulates the expression only of NFATcl, while it signals through both NFATcl and NFkappaB. (C) 2004 Elsevier Inc. All rights reserved.