c-src is known to play an essential role in osteoclastogenesis. We studied the regulatory mechanism as well as the significance of c-src induction in RANKL-induced differentiation of mouse monocytic RAW264 cells to TRAP-positive-multinucleated cells. We determined the genomic organization of the 5'-terminal region of mouse c-src. Mutational and biochemical analyses in the region 0.9 kb upstream of the transcription start site revealed that c-Fos and JNK pathways, in addition to NF-kappaB, participate in c-src induction in response to RANKL. On the other hand, when the expression of c-.src was suppressed by introducing antisense src, the number of multinucleated cells formed was significantly reduced. Together, these findings show that the expression of c-src is under the control of AP-1 and NF-kappaB in the differentiation of RAW264 cells and that c-src plays an essential role at the stage of progression to multinucleated cell formation. (C) 2004 Elsevier Inc. All rights reserved.