The CheckMate? Mammalian Two-Hybrid System provides positive control vectors that encode and express two proteins known to interact in vivo (13). MyoD is expressed in skeletal muscle and is a myogenic regulatory protein (14,15). The Id protein acts as a negative regulator of myogenic differentiation (16). MyoD and Id are members of the helix-loop-helix family of nuclear proteins. The pBIND-Id Control Vector contains a segment of murine Id cDNA (amino acids 29–148; 13,16) cloned in-frame with the GAL4 DNA-binding domain of the pBIND Vector (Figure 6). The pACT-MyoD Control Vector contains a segment of murine MyoD cDNA (amino acids 1–318; 13,15) cloned in-frame with the VP16 activation domain of the pACT Vector (Figure 6).