PDGFD and cancer: For liver cancer without viral infection, we developed a regression model for the unimodal distribution of the occurrence rates of liver cancers (ypx) as follows:(8)ypx=−35.6∗f1x−7.68∗f2x+1.3∗10−2∗f1x∗f2x+1.38∗10−1∗f1x∗f3x−3.21∗103where f1x is the age-dependent distribution of cancer risk, f2x and f3x are regression models for the age-dependent available levels of the identified growth signals TGFβ1 and PDGFD, specifically needed by liver cancer, respectively.