iRFP670 is a dimeric infra-red fluorescent protein with an excitation peak of 643nm and an emission peak of 670nm in vitro. It is an artificial derivative of the bacteriophytochrome encoded by the Rhodopseudomonas palustris RpBphP6 gene (PMID:23770755). In order to fluoresce, iRFP670 requires a biliverdin (linear tetrapyrrole) cofactor to act as the chromophore. This can be supplied in a transgenic animal by including a transgenic construct that encodes a heme oxygenase (HO).