Described herein is a G-protein coupled receptor (GPCR)-directed mutational analysis of tumor DNA obtained from melanoma tissue samples. The GPCR gene glutamate receptor, metabotropic 3 (GRM3) was identified as the most highly mutated GPCR gene in this screen. Functional characterization of GRM3 mutants revealed that these mutants promote activation of MEK, anchorage-independent cell growth and metastasis. Thus, provided herein are methods of diagnosing a subject as having melanoma, or susceptible to developing melanoma, by detecting the presence of at least one mutation in GRM3. Also provided are methods of treating a subject with melanoma by detecting the presence of at least one mutation in GRM3 and administering an appropriate therapy. Further provided are methods of selecting a subject diagnosed with melanoma as a candidate for treatment with a GRM3 inhibitor, an MEK inhibitor, or both, by detecting the presence of at least one mutation in GRM3.