Abstract Objective: We sought to define in colorectal cancer (CRC) the function of Shoca-2 (a.k.a. SH2D4A), an until now uncharacterized SH2 domain-containing protein. Methods: Tandem affinity purification, immunoprecipitation, site-specific mutagenesis, shRNA expression targeting and gene expression analysis were used to characterize binding partners of human Shoca-2 and identify the signaling pathway(s) where this adapter protein is involved. To assess Shoca-2 function on cell growth, colony formation assays and cell cycle analyses were carried out. CRC biopsies were analyzed by immunohistochemistry and molecular genetic methods for the presence of Shoca-2. Results: Here we demonstrate that Shoca-2 forms a complex with EGFR and STAT3. In response to EGF signaling, a Shoca-2/STAT3 dimer dissociates from EGFR and recruits the serine/threonine phosphatase PP1beta to the complex. Consequently, STAT3 activity is repressed and the EGF signaling pathway ceases to be activated. A lack of Shoca-2 expression secondary to a genomic loss of the locus is frequently observed in metastatic colorectal cancer and correlates with an increase in phosphorylated STAT3 in these cells. Conversely, the ectopic expression of wild type Shoca-2 but not of a mutant form defective in its ability to bind to PP1beta suppresses STAT3-dependent tumor cell growth. Conclusion: Our results identify Shoca-2 as a tumor suppressor that acts upon EGF stimulation as an adaptor protein to target STAT3 for dephosphorylation. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3081.