Actin and myosin are the two major cytoskeleton proteins implicated in cellular movement, secretion, phagocytosis, and kinesis. Actin is one of the most conserved cellular protein. At least 6 actin isoforms have been identified by protein sequence analyses. Four actin isoforms represent the differentiation markers of muscle tissues. There are three a-actins: a-skeletal, a-cardiac, and a-smooth muscle), one b-actin (b-non-muscle), and two g-actins (g-smooth muscle and g-non-muscle). Actin isoform are ~90% conserved, except in the N-terminal 18aa (50-60%homology). Beta-actin protein and mRNA levels are often used as a reference for comparing changes in cellular protein/mRNA levels by Western or Northern blots.