Adult Merkel cell precursors express Atoh1 and are unipotent. In this and all figures, dosing and harvest paradigms are shown above the pertinent panels. (A–B′) Xgal staining of hairy skin (A and B) and whisker follicles (A′ and B′) shows the presence of labeled cells 3 (A and A′; n = 3 mice) and 9 (B and B′; n = 1 mouse) mo after tamoxifen. Insets in A and B are individual touch domes. (A′ and B′) Counterstain is Nuclear Fast red. (C–D‴) Touch domes (C–C‴) and whisker follicles (D–D‴) immunostained for K8 and β-Gal. (E) Percentages of K8⁺ cells that coexpress β-Gal at 3 (n = 3) and 9 (n = 1) mo after tamoxifen (TMX). Error bars show SEM. (F–G″) Hairy skin from a tamoxifen-treated P28 Atoh1^CreER-T2/+;ROSA^tdTomato mouse immunostained for K8 (n = 3 mice). tdTomato⁺ cell (arrows) that appears to be K8⁻ at exposure times that identify other K8⁺ cells (F–F″) in fact expresses low levels of K8 (G–G″). Bars: (A and B, main images) 1 mm; (A and B, insets) 100 µm; (A′ and B′) 100 µm; (C–G″) 50 µm.