function, we carried out gene transfer studies using a plasmid in which Vcsa1 was expressed from a cytomegalovirus promoter (pVAXVcsa1). This plasmid was injected intracorporally into old rats, and the effect on physiology of corporal tissue was analysed by intracorporal/blood pressure (ICP/BP) measurement and histological analysis, and compared with the effects of a positive control plasmid (pVAX-hSlo, which we previously reported to restore erectile function in diabetic and ageing rats) and a negative control plasmid (pVAX). RESULTSIn each rat model of ED there was a significant down-regulation of the Vcsa1 transcript of at least 10-fold in corporal tissue. Remarkably, intracorporal injection with 80 µ g pVAX-Vcsa1 caused priapism, as indicated by visible prolonged erection, histological appearance, and elevated resting ICP/BP. Lower doses of pVAX-Vcsa1 (5 and 25 µ g) increased ICP/BP over that in untreated controls. CONCLUSIONThese results show that Vcsa1 has a role in erectile function and might be a molecular marker for organic ED. The role of Vcsa1 in erectile function suggests that it could represent a novel therapeutic target for treating ED.