The search terms included a combination of the following keywords and their variants: “gemcitabine,” “resistance,” “mechanism,” “tumor microenvironment,” “RRM2,” “RRM1,” “human equilibrative nucleoside transporter 1 (hENT1),” “dCK,” “nanoparticle,” “combination therapy,” “immunotherapy,” along with specific cancer types such as “pancreatic cancer,” “non-small cell lung cancer,” and “bladder cancer.” The search was limited to articles published in English. Here, SLC29A1 is linked to pancreatic neoplasm.