ENG and neoplasm: Rare, proliferating ECs were notably detected in the medium-large veins below the activated epicardium (Supplemental Figure 2B), and veins, but not arteries, within the border zone, expressed endoglin/CD105 (Supplemental Figure 2, C–E), previously implicated in extracellular matrix (ECM) remodeling, intimal angiogenesis, and tumor neovascularization (19), suggesting a higher propensity for venous, compared with arterial, sprouting.