Heterologous protein expression is useful for the production of recombinant proteins for both therapeutic and basic research purposes. Regulated expression systems can be useful for maximizing production levels without inhibiting cell proliferation, particularly if the protein is toxic to the cell. Inducible expression systems are usually regulated at the level of transcription by inducer or repressor molecules, and often suffer from leaky background expression and/or poor inducibility. In this issue, Boorsma et al. ( p. 429) describe a new system, based on transcription of a heterologous gene by a modified temperature-sensitive RNA-dependant RNA replicase from alphavirus. Their modified viral replicase is only active at temperatures below 35°C, and exhibits low background and high inducibility of various reporter genes in both transiently and stably transfected cells.