mKeima is a monomeric, pH-sensitive red fluorescent protein. It is an artificial derivative of a non-fluorescent GFP-like protein encoded by a cDNA clone isolated from the stony coral Montipora sp. (PMID:16648840). mKeima has an emission spectrum that peaks at 620nm and does not change with pH. It has a bimodal excitation spectrum with peaks at 440 and 586nm, corresponding to the neutral and ionized states of the chromophore respectively. The neutral state predominates at high pH and the ionized state predominates at low pH, with a pKa value of 6.5, and thus the excitation spectrum changes according to pH (PMID:16648840, PMID:19722611, PMID:21867919). Dual-excitation ratiometric pH measurements can be performed using mKeima (550/438) and the cumulative fluorescent readout can be used to quantify autophagy at a single time point (PMID:21867919).