This Phase 1/2, open-label, multicenter study is conducted in patients with previously treated selected solid tumors, including non-small cell lung cancer (NSCLC), small cell lung cancer (SCLC), high-grade neuroendocrine cancer of any primary site, diffuse large B-cell lymphoma (DLBCL), and tumors with L-MYC or N-MYC amplification. Patients receive escalating doses of a GSPT1 molecular glue degrader MRT-2359 to determine safety, tolerability, maximum tolerated dose (MTD) and/or recommended Phase 2 dose (RP2D) of MRT-2359. Once the MTD and/or RP2D is identified, additional patients enroll to Phase 2 study, which includes molecular biomarkers stratification or selection, namely expression or amplification of L-MYC and N-MYC genes, hormone receptor positive (HR)-positive, human epidermal growth factor 2 (HER2)-negative breast cancer and prostate cancer.