Hepatic expression of cell cycle regulators and determination of hepatocellular size in myostatin-null mice. Basal hepatic Cyclin D1, Cyclin E, and Cdc25A mRNA expression (A) and hepatocellular nuclear density (number of nuclei per five ×400 high-powered-fields [HPF], as a surrogate for hepatocyte size) (B) in 3-month-old wild-type, heterozygous, and myostatin-null mice. *P = 0.1 versus wild type; P = 0.02 versus Het; **P = 0.2 versus wild type; P = 0.05 versus Het; ***P = 0.01 versus wild type, Het.