Dopamine and cAMP-regulated phosphoprotein Mr 32,000 (DARPP-32), also known as phosphoprotein phosphatase-1 regulatory subunit 1B (PPP1R1B), was initially discovered as a substrate of dopamine-activated protein kinase A (PKA) in the neostriatum in the brain. While phosphorylation at Thr-34 by PKA converts DARPP-32 into a potent inhibitor of protein phosphatase 1 (PP1), phosphorylation at Thr-75 transforms DARPP-32 into an inhibitor of PKA. Through regulation of DARPP-32 phosphorylation and modulation of protein phosphatase and kinase activities, DARPP-32 plays a critical role in mediating the biochemical, electrophysiological, and behavioral effects controlled by dopamine and other neurotransmitters in response to drugs of abuse and psychostimulants. Altered expression of DARPP-32 and its truncated isoform (t-DARPP), specifically in the prefrontal cortex, has been associated with schizophrenia and bipolar disorder. Moreover, cleavage of DARPP-32 by calpain has been implicated in Alzheimer's disease. Amplification of the genomic locus of DARPP-32 at 17q12 has been described in several cancers. DARPP-32 and t-DARPP are frequently overexpressed at the mRNA and protein levels in adenocarcinomas of the breast, prostate, colon, and stomach. Several studies demonstrated the pro-survival, pro-invasion, and pro-angiogenic functions of DARPP-32 in cancer. Overexpression of DARPP-32 and t-DARPP also promotes chemotherapeutic drug resistance and cell proliferation in gastric and breast cancers through regulation of pro-oncogenic signal transduction pathways. The expansion of DARPP-32 research from neurotransmission to cancer underscores the broad scope and implication of this protein in disparate human diseases.