Table 1.

Selected genetically encoded pH indicators (GEPIs)

GEPI	ΔF/F, pH 5.5 to pH 7.4	Brightness at pH 5.5 (mM−1 cm−1)a	Brightness at pH 7.4 (mM−1 cm−1)b	pKa	Refs.
EGFP	5.0	6.4	32	6.0	30,124
Ecliptic pHluorin (EP)	50	0.070	3.5c	7.1	29,30
Superecliptic pHluorin (SEP)	50	0.44	22d	7.2	30
pHTomato	2.0	24	48	7.8	31,125
td-mOrange2	3.5	10	35	6.5	39,126,127
pHoran4	17	3.2	55	7.5	31
pHuji	20	0.34	6.8	7.7	31

^aBrightness at pH 5.5 was calculated from brightness at pH 7.4 (below) and reported ΔF/F.

^bBrightness at pH 7.4 was directly measured as product of peak extinction coefficient and quantum yield of mature protein measured at pH 7.4, unless otherwise indicated.

^cBrightness at pH 7.4 was estimated from relative brightness of EP upon 488-nm excitation at pH 7.4 vs. maximal observed brightness (at pH 8.5), with maximal brightness assumed to be similar to that of the parental wild-type GFP, which has a similar excitation spectrum shape. Brightness of wild-type GFP at 488 nm was derived from the published absorbance spectrum, extinction coefficient, and quantum yield¹²⁴. Brightness of EP in cells relative to EGFP and SEP may be disproportionally low owing to absence of the F64L folding mutation.

^dBrightness at pH 7.4 was estimated from relative brightness of SEP upon 488-nm excitation at pH 7.4 vs. maximal observed brightness (at pH 9.5), with maximal brightness assumed to be similar to that of the parental EGFP, which has a similar excitation spectrum shape. Published measurements were used for the brightness of EGFP at 488 nm (ref. 124). Values were rounded to two significant digits.