cpFLIPPi-6.4m is a genetically encoded fluorescence resonance energy transfer (FRET) sensor for inorganic phosphate (Pi). It contains an inorganic phosphate binding protein (PiBP) from a Synechococcus sp. pstS gene, flanked by ECFP and circularly permuted Venus (cpVenus), which act as FRET donor and acceptor respectively. Short linker sequences are present between each of the fluorescent proteins and the PiBP sequence and a number of mutations have been introduced into the PiBP coding sequence. Binding of inorganic phosphate (Pi) to the PiBP results in conformational changes that increase the distance between the FRET donor (ECFP) and acceptor (cpVenus), reducing FRET (PMID:25624397).