Protein phosphatase type 2A (PP2A), an essential protein serine/threonine phosphatase conserved in all eukaryotes, regulates fundamental cellular activities such as DNA replication, transcription and translation, metabolism, cell cycle progression and cell division, apoptosis, development and various signal transduction pathways. The core enzyme consists of a catalytic C and a regulatory A (also known as PR65) subunit and each subunit exists as a and b isoforms. Another regulatory B subunit is composed of four different families of proteins with no sequence similarity between them. The B or PR55 family is composed of a, b, g and d isoforms; BI or PR61 family is composed of a, b, g, d and e isoforms; BII family consists of PR72, PR130, PR59 and PR48; BIII family has two members, striatin (PR110) and SG2NA (PR93). These B subunits bind to the same region of the A subunit and their interaction with the core enzyme is mutually exclusive. The diversity of the regulatory subunits, especially the B subunits, allows PP2A to perform its diverse functions. The function of PP2A is regulated by expression, localization, holoenzyme composition and post-translational modification. It has been reported that PP2A is phosphorylated at Tyr307 by Src in response to EGF or insulin, which inactivates PP2A activity. Reversible methylation on the carboxyl group of Leu309 of PP2A has been observed. Methylation alters the conformation of PP2A, as well as its localization. PP2A methylation also regulates its association with B regulatory subunits.