The scaffold protein spinophilin (SPN, PPP1R9B) is a regulatory subunit of phosphatase-1a located at 17q21.31. This region is frequently associated with microsatellite instability and LOH and contains a relatively high density of known tumor suppressor genes (such as BRCA1), putative tumor suppressor genes and several unidentified candidate tumor suppressor genes located distal to BRCA1. Spn is located distal to BRCA1, and we have previously shown that the loss of Spn contributes to human tumorigenesis in the absence of p53 function. In this work, we explore the role of Spn as putative tumor suppressor in in vivo models using genetically modified mice. Spn-knockout mice had decreased lifespan with increased cellular proliferation in tissues such as the mammary ducts and early appearance of tumors, such as lymphoma. Furthermore, the combined loss of Spn and mutant p53 activity led to increased mammary carcinomas, confirming the functional relationship between p53 and Spn. We suggest that Spn may be a novel tumor suppressor located at 17q21.