(A) Immunostaining of P5 WT cochlea shows Sox2 expression in Hensen’s cells, Deiters’ cells, pillar cells, and the lateral portion of the greater epithelial ridge. (B) Whole-mount preparation of cochlea from P4 Sox2^CreERT2/+ R26R^tdTomato/+ mice given tamoxifen on P2, showing tdTomato expression in supporting cells and some hair cells. (C) GFP⁺ supporting cells in the P5 Sox2^GFP/+ cochlea. (D) Schematic of EdU administration to Sox2^CreERT2/+ mice, Sox2^GFP/+ mice, and WT littermates (once daily, P2–P4). haplo, haploinsufficient. (E) qPCR showed a significant reduction of Sox2 expression in Sox2^CreERT2/+ cochleae compared with expression in WT littermates. (F) Confocal images show no EdU⁺ hair cells or supporting cells in the P5 WT cochlea. EdU labeling was seen in cells in the lesser epithelial ridge and greater epithelial ridge. (G) Sox2^CreERT2/+ cochlea contained occasional extranumerary hair cells adjacent to inner hair cells (arrowheads). Extranumerary hair cells were noted in all cochlear turns of Sox2^CreERT2/+ mice. Image shows EdU⁺ supporting cells (chevrons) in the apical turn. No EdU⁺ hair cells were noted. (H) Quantification of extranumerary hair cells in WT, Sox2^CreERT2/+, and Sox2^GFP/+ cochleae. (I) Quantification of EdU⁺ cells in WT, Sox2^CreERT2/+, and Sox2^GFP/+ cochleae. (J) P28 Sox2^CreERT2/+ mice had normal ABR thresholds, comparable to those of their WT littermates. DC, Deiters’ cell; GER, greater epithelial ridge; HC, hair cell; IHC, inner hair cell; IP, inner pillar cell; IPhC, inner phalangeal cell; LER, lesser epithelial ridge; OHC, outer hair cell; OP, outer pillar cell; Ortho, orthogonal view; PC, pillar cell; SC, supporting cell. Data represent the mean ± SD. *P < 0.05 and **P < 0.01, by 2-tailed Student’s t test. n = 3–8. Scale bar: 20 μm.