RecruitingPhase 2Drug trialStudy of All-Trans Retinoic Acid (ATRA) and Cemiplimab in Patients With Advanced LeiomyosarcomaA phase 2 study assessing the efficacy of all-trans retinoic acid (ATRA) and Cemiplimab in patients with metastatic/locally advanced - unresectable leiomyosarcoma (LMS) who have progressed standard-of-care therapy. Patients will be enrolled in cohorts according to a Bayesian Optimal Phase II design (BOP2). Study treatment will consist of ATRA at a starting dose of 150 mg/m2/day for 3 days orally prior to each cycle of Cemiplimab 350 mg IV q3 weeks for three cycles and then Cemiplimab monotherapy until the progress of disease or unacceptable toxicities develops.