AbstractWe have isolated and sequenced a cDNA (mrfms) encoding ratc-fmsgene (CSF-1 receptor) from proliferating L6α1 myoblasts. The predicted amino acid sequence was highly identical with thec-fmsprotein found in monocytes and macrophages (98, 76 and 84% identity from mouse, cat and humanc-fmsproteins, respectively). The mechanisms responsible for the regulation of mrfms gene expression during myogenesis were examined. Mrfms products were observed during proliferation of L6α1 myoblasts and were downregulated during differentiation. Run-on transcription assays demonstrated that the mrfms gene was transcriptionally active only in undifferentiated myoblasts. These findings suggested that mrfms levels in L6α1 myoblasts are controlled by transcriptional mechanisms. The half-life of mrfms transcripts was found to be at least 5 hr while inhibition of protein synthesis with cycloheximide (CHX) decreased this half-life to 30 min without changes in the rate of mrfms gene transcription. In addition oncogenic transformation of L6al myoblasts by thev-frnsinduced constitutive upregulation of mrfms mRNAs, and nuclear run-on assays demonstrated that mrfms transcription was not growth-factor dependent. Furthermore, these findings with others previously published indicate that mrfms gene products may play a role in the normal and neoplastic growth of muscular cells.