aeq corresponds to a naturally occurring Ca[2+]-sensitive luminescent protein from Aequorea victoria. aeq consists of a 22kD apoprotein (UniProtKB:P07164) bound to coelenterazine, a hydrophobic luminophore. Luminescence is activated by binding to Ca[2+], and this property can be used to monitor Ca[2+] levels in vivo (reviewed in PMID:24371311).