TTR and cancer: Sun et al. (2015); Vrba and Futscher (2018), and other researchers used genes specifically expressed in a limited number of tissues as plasma DNA methylation markers for noninvasive prenatal, cancer, or transplantation assessments. Their findings suggested that genes specifically expressed in a limited number of cell types could also serve as CTS genes. Panina et al. (2020) summarized the CTS gene databases for mice and humans, including Labome, CellFinder (Stachelscheid et al., 2014), CellMarker (Zhang et al., 2019), PanglaoDB (Franzén et al., 2019), and SHOGoiN (Hatano et al., 2011).