Nucleoside analogues (NA) constitute an important class of pro-apoptotic antimetabolites used in the treatment of hematological malignancies, solid tumors, and infectious diseases such as AIDS and hepatitis (1, 2). These therapeutic compounds include various pyrimidine and purine analogues (1) that mimic physiological nucleosides in terms of uptake into the cell and metabolism. Nucleosides and nucleoside derived drugs are hydrophilic molecules and diffuse slowly across cell membranes, thus, specific membrane proteins mediate their translocation from the extracellular milieu into the cytoplasm and, in some cases, from the cytoplasm towards the extracellular milieu. This cellular uptake occurs via nucleoside-specific membrane transporters (NT), belonging to the solute carrier families SLC28 and SLC29, encoding the concentrative (hCNT), which are Na+-dependent, and equilibrative (hENT), which are Na+-independent transporters, respectively. NA entrance into cells contribute to drug bioavailability and hence to cytotoxicity or antiviral capacity of nucleoside-derived drugs. Among NT, hCNT proteins mediate concentrative high-affinity nucleoside influx by coupling substrate translocation to the inwardly directed electrochemical sodium gradient. This family comprises three members: hCNT1, hCNT2, and hCNT3. hENT proteins mediate facilitated, sodium-independent nucleoside-derived drug translocation. hENT1 and hENT2 are major determinants of equilibrative drug transport across the plasma membrane. Resistance to NA treatments is usual in clinics. A key event in resistance is drug availability and entrance into the cells through NT. This is why the expression and activity of those molecules at the plasma membrane are essential. Regulation of nucleoside transport activity in an isoform-specific manner is a suitable approach to modulate nucleoside-derived drug bioavailability. In this sense, it was shown that hCNT3 protein is retained intracellularly in a subpopulation of chronic lymphocytic leukemia (CLL) cases, showing that those patients with elevated levels of intracellular hCNT3 achieved a lower complete response rate after the purine analog fludarabine therapy (3). We have recently reported that all-trans retinoic acid (ATRA) induces hCNT3 trafficking to the plasma membrane, causing an increase in hCNT3 transporter activity and consequently fludarabine entrance in all fludarabine-resistant CLL samples analyzed (4). This effect was partially mediated by TGF-β1 by a mechanism that was dependent on the activation of the mitogen-activated protein (MAP) kinases p38 and ERK1/2 (5). Moreover, a reversion of fludarabine insensitivity was achieved in fludarabine-resistant CLL samples after ATRA or TGF-β1 treatment. We have also found that induced traffic of NT to the plasma membrane seems to be involved in the combination therapy involving interferon α (IFNα) and the broad spectrum antiviral ribavirin, a purine nucleoside derivative drug. IFNα was shown to significantly increase the effect of ribavirin over the viral load (6). Although in basal conditions hENT1 is the main ribavirin transporter (7), no studies have evaluated the role of hCNT2, a putative ribavirin good transporter, in the presence of IFNα. We recently demonstrated that IFNα induces a transient increase in hCNT2 activity within 15 minutes in HHL5-hepatocytes cells. This increase seems to be dependent on the translocation of hCNT2 containing vesicles to the plasma membrane by an ERK1/2 activation and microtubule integrity (Pinilla-Macua, Fernández-Calotti and Pastor-Anglada, submitted). It is clear that NT expression is variable and that the available evidence supports a putative role for NT in nucleoside-derived drug bioavailability and responsiveness. NT expression pattern analysis might be useful as a biomarker of drug metabolism and action or as a predictor of response to therapy and patient outcome. Thus, major efforts should be made to determine the biological basis for the specific regulatory properties of NT trafficking as a way to improve cancer and viral therapies.