GCaMP1.6 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, with the additional mutations V163A and S175G (PMID:16159115).