Using rat and human tissues, CNT and ENT cDNAs were cloned and their functional characterisation was determined using Xenopus oocytes. This study revealed that CNT1, CNT3 and ENT2 but not CNT2 or ENT1 transported AZT but with a much lower affinity than endogenous nucleosides. In addition, the influx of AZT into the CSF through choroid epithelial cells was found to be unaffected by endogenous nucleosides (Yao et al., 2001; Strazielle et al., 2003). CNT has also been implicated in the transport of ddI across the BBB (Li et al., 2001). However, the role of human CNT1 as a uptake transporter for ddI and stavudine (d4T) has remained controversial, as some researchers have described such a function for hCNT1 whereas others (Minuesa et al., 2008; Chishty et al., 2004; Chang et al., 2004; Cano-Soldado et al., 2004) have argued that hCNT1/2 (and even hENT1) have a low affinity for these drugs due to the absence of the 3′OH group in the ribose ring of the structure of nucleoside analogs. Conversely, hCNT3 appears to be an efficient transporter of AZT, ddC and ddI (Minuesa et al., 2008).