GCaMP1.3 is a genetically encoded fluorescent Ca[2+] indicator. It is a fusion protein composed of the calmodulin-binding peptide M13, a circularly permuted fluorescent protein and calmodulin. The circularly permuted fluorescent protein is interrupted at residue 145, and is based on EGFP. For a comparison of GCaMP1.3 with other Ca[2+] indicators, see FBrf0187903.