CA4 and neoplasm: Notably, few cells of the two subclusters expressed markers for human heart capillary ECs (e.g., RGCC and CA4), arterial ECs (e.g., SEMA3G), or venous ECs, for example, ACKR1,15 whereas both subclusters expressed the pan‐EC marker CDH5 and markers for endocardial ECs (e.g., NPR3 and SMOC1; Figure S8A), suggesting endocardial EC‐like differentiation of tumour cells.