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We report that gene dosage, or the ratio of nuclei from two cell types fused to form a heterokaryon, affects the time course of differentiation-specific gene expression. The rate of appearance of the human muscle antigen, 5.1H11, is significantly faster in heterokaryons with equal or near-equal numbers of mouse muscle and human fibroblast nuclei than in heterokaryons with increased numbers of nuclei from either cell type. By 4 d after fusion, a high frequency of gene expression is evident at all ratios and greater than 75% of heterokaryons express the antigen even when the nonmuscle nuclei greatly outnumber the muscle nuclei. The kinetic differences observed with different nuclear ratios suggest that the concentration of putative trans-acting factors significantly influences the rate of muscle gene expression: a threshold concentration is necessary, but an excess may be inhibitory.