The mLexA::VP16::NFAT driver encodes a calcium ion-responsive transcriptional activator designed for use in a binary expression system. It is composed of the lexA gene (ECOGENE:EG10533) from Escherichia coli carrying mutation(s) that inactivate its nuclear localization signal, a transcriptional activation domain from the Herpes simplex virus Type 1 VP16 gene (UniProtKB:P06492) and the regulatory domain of the Homo sapiens NFATC1 gene (HGNC:7775). The NFATC1 regulatory domain contains a nuclear localization signal whose function is tightly controlled by the Ca[2+]/calmodulin dependent phosphatase calcineurin (PMID:12975316). The lexA DNA-binding domain can bind specifically to the lexAop DNA sequence, but in the mLexA::VP16::NFAT driver, the presence of the NFATC1 regulatory domain means that the driver enters the nucleus and activates transcription only in response to a sustained increase in the level of intracellular calcium ions. The exact sequence of the driver may differ depending on the particular transgene or modified endogenous locus being used. In particular, the extent of the NFATC1 sequence that is present in addition to its regulatory domain may vary, which can affect the baseline activity of the protein. The mLexA::VP16::NFAT driver can be used to report sustained neural activity and has been used as an activity-dependent neural circuit tracer in vivo (FBrf0217864).