Ion channels are increasingly being linked to cancer and tumour progression. Here we describe a voltage-gated, potassium selective channel (Eag1, Kv10.1) with novel electrophysiological properties, whose normal physiological function is yet unknown but which shows oncogenic transforming potential if expressed ectopicaly. One of the most characteristic properties of the channel, its ionic selectivity, is modulated during cell-cycle transitions. Strikingly, the expression of the human Eag1 is restricted to brain, but it is also present in several tumour-derived cell lines. More importantly, the protein can be detected in more than 75% of human tumour samples, while the corresponding normal tissues are devoid of the channel. Experiments under in vitro conditions have demonstrated decreased proliferation of Eag1-expressing cells by inhibition of expression and/or function of this channel. This inhibition of Eag1 is accomplished using RNA interference, functional anti-Eag1 antibodies, or (unspecific) EAG1 channel blockers. We have also used in vivo models to visualise the distribution of Eag1 in tumour-bearing mice using specifically designed recombinant antibodies. We conclude that Eag1 is a widely distributed tumour marker with diagnostic and therapeutic potential.