Reversible protein methylation may be another posttranslational modification that serves an important role in modulating signal transduction pathways. Not only does protein phosphatase 2A (PP2A), a key regulator of many signal transduction cascades, undergo this modification, but the methylation process itself may be regulated by various cellular stimuli or states. Mumby describes how PP2A is regulated and how methylation may alter its subcellular targeting and substrate specificity by influencing its interaction with regulatory subunits.