A method for identifying a candidate agent for the treatment of hearing loss or balance loss associated with a loss of cochlear or vestibular hair cells, the method comprising: (a) maintaining a population of inner ear supporting cells under conditions sufficient to produce an expanded population of inner ear supporting cells, wherein the population of inner ear supporting cells are isolated from a transgenic mouse having two or more recombinant nucleic acid molecules stably integrated into the genome of the mouse, comprising at least a first recombinant nucleic acid molecule that comprises a first reporter gene under the control of a regulatory element of an inner ear supporting cell marker selected from the group consisting of Lgr5, Sox2, p27, Prox1, FGFR3, Glast, and Lfng, and a second recombinant nucleic acid molecule that comprises a second reporter gene under the control of a regulatory element of an inner ear hair cell marker selected from the group consisting of Atoh1, Myo7a, Cdh23, Pcdh15, Myo6, Myo1c, Tmc1, and Cav 1.3, wherein the first reporter gene is different from the second reporter gene; (b) administering a test compound to the expanded population of inner ear supporting cells; (c) detecting the expression levels of the first reporter gene and the second reporter gene in the expanded population of inner ear supporting cells in the presence of the test compound; and (d) selecting as a candidate agent for the treatment of hearing loss or balance loss a test compound that increases the expression level of the first reporter gene compared to the expression level of the first reporter gene in the absence of the test compound, and/or increases the expression level of the second reporter gene compared to the expression level of the second reporter gene in the absence of the test compound. (a) maintaining a population of inner ear supporting cells under conditions sufficient to produce an expanded population of inner ear supporting cells, wherein the population of inner ear supporting cells are isolated from a transgenic mouse having two or more recombinant nucleic acid molecules stably integrated into the genome of the mouse, comprising at least a first recombinant nucleic acid molecule that comprises a first reporter gene under the control of a regulatory element of an inner ear supporting cell marker selected from the group consisting of Lgr5, Sox2, p27, Prox1, FGFR3, Glast, and Lfng, and a second recombinant nucleic acid molecule that comprises a second reporter gene under the control of a regulatory element of an inner ear hair cell marker selected from the group consisting of Atoh1, Myo7a, Cdh23, Pcdh15, Myo6, Myo1c, Tmc1, and Cav 1.3, wherein the first reporter gene is different from the second reporter gene; (b) administering a test compound to the expanded population of inner ear supporting cells; (c) detecting the expression levels of the first reporter gene and the second reporter gene in the expanded population of inner ear supporting cells in the presence of the test compound; and (d) selecting as a candidate agent for the treatment of hearing loss or balance loss a test compound that increases the expression level of the first reporter gene compared to the expression level of the first reporter gene in the absence of the test compound, and/or increases the expression level of the second reporter gene compared to the expression level of the second reporter gene in the absence of the test compound.