Beta-actin is commonly used to normalize molecular expression studies due to its high conservation as an endogenous housekeeping gene. However, recent studies have shown that beta-actin expression can change in response to biochemical stimuli, during growth and differentiation, and in disease states. As can be expected, these phenomena substantially compromise the use of beta-actin as an internal reference marker. Under these conditions, varying expression of beta-actin likely indicates changed function for this maintenance molecule. Further studies exploring the function of actin in these environments will probably lead to a new integrative understanding of the roles of this housekeeping gene.