The level of endogenous biomarkers (e.g., CPs) may change under disease states, such as kidney transplantation (Suzuki et al., 2019), chronic kidney disease (Takita et al., 2022), liver injury (Lin J. et al., 2023), and rheumatoid arthritis (Ono et al., 2021), which are associated with pathological changes in transporter activity. The gene discussed is CAD; the disease is chronic kidney disease.