Protein phosphatase 2A (PP2A) is a protein serine/threonine phosphatase that regulates the activities of key rate-limiting enzymes and proteins involved in diverse cellular processes (1–4). However, although much is known about the subunit composition and sequence of different forms of the enzyme (1–7), little information is available on its regulation. Nonetheless, evidence has emerged that the catalytic subunit of PP2A is subject to posttranslational modifications including phosphorylation on threonines (8–10) and tyrosines (11), and methylation at Leu³⁰⁹ (12–14). Phosphorylation at either threonine or tyrosine residues inactivates PP2A (8–10). By contrast, methylation appears to activate the phosphatase (14).