Sensory axons fail to innervate whisker follicles in Brn3a null mice. The whisker follicles of Brn3a^+/− (A, C,E) and Brn3a^−/− (B,D, F) embryos were examined in detail at E13.5 for the expression of β-gal by activity staining (A, B) and immunohistochemistry for β-gal protein (C, D). At this stage, most of the follicles of Brn3a^+/− mice (and Brn3a^+/+ mice; data not shown) were completely encircled by β-gal-containing sensory axons, whereas follicles of Brn3a^−/− mice were mostly not encircled. The follicles of both embryos appeared normal by hemoxylin–eosin histology (E, F).