GWAS loci frequently deleted in canine osteosarcoma [18] implicate candidate genes such as the tumor suppressors PPP1R13L (encoding the TP53-inhibitor iASPP) and PTPN18 (inhibitor of proliferation through the FGF/growth factor signaling/PI3K and RB/cell cycle pathways [27]). Here, RB1 is linked to osteosarcoma.