jRGECO1a is a genetically encoded red fluorescent Ca[2+] indicator. It is a fusion protein composed of a calmodulin-binding peptide, a circularly permuted red fluorescent protein and calmodulin. The circularly permuted fluorescent protein is based on mApple and contains the amino acid substitutions Q306D and M339F relative to its progenitor R-GECO1. jRGECO1a provides an improved signal-to-noise ratio (FBrf0232227).