A strategy for increasing the expression of the factors regulating myogenesis was developed based upon the observation that increased amounts of regulatory factors could overcome the inhibition of differentiation produced by 5-bromodeoxyuridine (BUdR). L6 rat myoblasts were subjected to multiple cycles of cloning in progressively increasing concentrations of BUdR. The first clones to differentiate were picked and replated for the next cycle of selection. After 28 cycles in BUdR, cells were isolated that could differentiate in the presence of 8 microM BUdR. Cell hybrids between myoblasts subjected to 21 cycles of selection (BU21 cells) and differentiation-defective myoblasts exhibited a high probability of differentiation, consistent with the hypothesis that BU21 cells were overproducing factor(s) involved in the decision to differentiate. The selection of cells able to differentiate in the presence of BUdR may provide a general approach for increasing the expression of the regulatory molecules controlling terminal differentiation.

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