pHuji is a pH-sensitive red fluorescent protein. It is an artificial derivative of mApple, containing a single additional amino acid change (K163Y) compared to the progenitor. pHuji has a pKa of 7.7 and shows a more than 20-fold change in fluorescence intensity from pH 5.5 to 7.5 in vitro. At pH 7.2 the excitation peak is 566nm and the emission peak is 598nm, and the emission peak shows negligible change with pH (PMID:25385186).