CD274 and neoplasm: •Metastatic <i>CSMD3</i> mutated HGSOC showed objective and sustained response to pembrolizumab.•The tumor was massively infiltrated by CD8<sup>+</sup> T cells while PD-L1 TPS was at 10%.•<i>CSMD3</i> mutated HGSOC showed up-regulation of <i>CCL5</i> and <i>CXCL9.</i>