SypHer is ratiometric pH sensor based on a circularly permuted YFP (cpYFP) (PMID:21224385). SypHer was generated by mutagenesis of HyPer (described in PMID:16554833). It consists of the regulatory domain of the Escherichia coli oxyR gene, into which cpYFP has been inserted (flanked on each side by a short linker) in between residues 205-206, with a C199S mutation relative to HyPer (to remove sensitivity to H[[2]]O[[2]]).