A transgenic construct containing Clomeleon, a fluorescent fusion protein containing CFP and topaz YFP, under the control of the mouse Thy1, thymus cell antigen 1, theta, promoter (Thy1.2) was injected into fertilized ICR mouse eggs. Founder line CLM12 was subsequently established. Transgenic mice express Clomeleon, a fluorescent fusion protein containing CFP and topaz YFP that acts as a ratiometric indicator for chloride ions, under the control of the mouse Thy1, thymus cell antigen 1, theta, promoter (Thy1.2). Transgene expression in founder line CLM12 is detected at high levels in the thalamus, cerebellar nuclei. No expression is detected in layers 1 and 2 of the neocortex, hippocampal regions CA2 and CA3, cerebellar purkinje cells, or retinal ganglion cells. In the presence of low chloride ion concentrations, YFP fluoresces as a FRET (Fluorescence Resonance Energy Transfer) acceptor for CFP. As the chloride ion concentration increases, YFP fluorescence decreases and CFP fluorescence increases. Using an appropriate calibration curve, the ratio of YFP and CFP fluorescence peaks indicate [Cl-].