TNFRSF21 and cancer: After attachment, intraluminal creep, and ultimately firm adherence to the endothelial lining, endothelium-bound cancer cells are able to proliferate and establish intravascular cancer microcolonies as metastatic foci (Al-Mehdi et al., 2000) or they undergo extravasation via necroptosis of endothelial cells through APP and DR6 (Strilic et al., 2016).