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A family of protein tyrosine phosphatases enriched within the central nervous system called striatal enriched phosphatase (STEP) has been implicated in the regulation of the N-methyl-d-aspartate receptor. STEP₆₁, a membrane-associated isoform located in the postsynaptic densities (PSDs) of striatal neurons, contains two transmembrane domains, two proline-rich domains, and a kinase-interacting motif. This study demonstrates that STEP₆₁ associates with Fyn, a member of the Src family kinases that is also enriched in PSDs. By using human embryonic kidney 293 cells for co-transfection, we determined that a substrate-trapping variant (STEP₆₁ CS) binds to Fyn but not to other members of the Src family present in PSDs. In a complementary experiment, myc-tagged Fyn immunoprecipitates STEP₆₁ CS. STEP₆₁ binds to Fyn through one of its proline-rich domains and the kinase-interacting motif domain …