CaMPARI(V398D) is a fluorescent Ca[2+] indicator that undergoes efficient and irreversible green-to-red conversion only in the presence of both elevated intracellular Ca[2+] levels and experimenter-supplied violet light. CaMPARI(V398D) consists of a nuclear export signal (Tag:NES(Unk)), a Ca[2+]-binding protein, a circularly permuted variant of mEos2 and an M13 peptide. The circularly permuted mEos2 sequence carries the following additional mutations relative to mEos2 : V2 insert, F34Y, S39T, A69V, L93M, I102Y, E140M, T143W, M146I, A160I, Y189V and H213Y (these mutations are present in its precursor, CaMPARI), plus a V398D substitution relative to CaMPARI, which results in CaMPARI(V398D) having a reduced affinity for Ca[2+] (FBrf0227755).