COL4A1 and breast cancer: Additionally, three subtypes of tumor-associated EC (tECs) were enriched in BC patients: EC4 cells, identified by ACKR1+ (19–21) and HLA-DRA+ expression (14, 22); EC5 cells, characterized by COL4A1+ and INSR+ expression; and EC6 cells, defined by IGFBP3+ and CXCL12+ expression.