Cell transfer studies of identical twin, where one twin is infected with HIV, are laying the groundwork for future gene therapies. Following several preliminary successes, approval for the first gene marker study in identical twins has just been granted by the US National Institutes of Health (NIH) Recombinant DNA Advisory Committee (RAC). The pioneering work leading up to this approval was described at the IIR Ltd conference on Gene Therapy, held in London, UK, last December.