ATM and undifferentiated carcinoma: Molecular profiling revealed high microsatellite instability (MSI-H) status, programmed death-ligand 1 (PD-L1) positivity, high tumor mutational burden, and an ATM mutation, guiding initiation of immunotherapy.<h4>Discussion</h4>Neoplasms in MD are rare and most commonly neuroendocrine in origin; undifferentiated carcinoma is exceedingly uncommon.