The twist gene encodes a transcription factor containing a conserved basic helix-loop-helix domain. During development, transcription factors of this type are normally associated with the induction of differentiation. Yet the expression pattern of the murine M-twist suggests an inhibitory role during muscle differentiation. Following stable transfection of myogenic mouse cells with an M-twist expression vector, 75% of M-twist-expressing clones were impaired in their ability to differentiate. In contrast, only 15% of control clones were unable to differentiate. Treatment with antisense oligonucleotides restored differentiation capacity in a concentration-dependent manner. Control oligonucleotides had no effect. These experiments show that the mouse twist gene can act as an inhibitor of muscle differentiation and that this inhibition is reversible.