We also show that this export pump does not confer resistance to various classes of cytotoxic drugs but does provide small but significant resistance to CdCl 2 and potassium antimonyl tartrate.

The in vivo transcription start sites of the human cystic fibrosis transmembrane conductance regulator gene ( CFTR ) and its murine homologue ( Cftr ) have been mapped in a range of tissues using the technique of 5' rapid amplification of cDNA ends (5' RACE). These are the first in vivo transcription start sites for CFTR or Cftr to be reported. Distinct, tissue-specific patterns of CFTR start site usage were identified in both mouse and human. In particular, striking variation in the position of the murine Cftr transcription start site was seen along the length of the intestinal tract; different start sites being utilized in ileum and in duodenum. In humans, distinct transcription start sites are utilized in adult and foetal lungs. In addition, a novel 5'-untranslated exon of murine Cftr , denoted exon -1, was identified and shown to be expressed exclusively in mouse testis. Expression of exon -1-containing Cftr transcripts was shown by mRNA in situ hybridization to be confined to the germ cells and to be regulated during spermatogenesis.