Hi, We would like to design a self-cleaving tag. It will be similar to the one Roger Tsien has developed but I hope to find one that works in the opposite way. Lin, M.Z. et al. A drug-controllable tag for visualizing newly synthesized proteins in cells and whole animals. Proc. Natl. Acad. Sci. USA 105, 7744–7749 (2008). Tsien used the NS3 protease which is inhibited by a cell-permeant drug. The tag will be cleaved in the absence of this drug. I would like to find a small and monomeric protease which is inactive until a drug or a co-factor is applied. Any suggestions will be highly appreciated. Best, Daniel