To study the process of disease relapse in previously responding tumors, we generated a cell line from a Brca1-mutant EPO-GEMM tumor that progressed following treatment with cisplatin and anti–PD-1 antibody and tested responsiveness to cisplatin therapy in vitro and to cisplatin and anti–PD-1 combination therapy in vivo following subcutaneous injection into syngeneic recipients. Here, EPO is linked to neoplasm.