Target genes with increased expression in the DRG of Brn3a null mice. (a) In situ hybridization confirms increased expression of Msc (musculin), NeuroD1, and Htr3a (serotonin receptor 3A) transcripts in the DRG of Brn3a wild-type and knockout embryos. Note that the expression of Msc in the surrounding musculature (arrows) is unchanged. (b) Immunofluorescence for the somatostatin-14 peptide shows increased expression concentrated in the dorsal root entry zone (arrows). The distribution of somatostatin-14 also reveals an abnormal accumulation of axons in the superficial dorsal horn in the Brn3a null mutant. All views show lower cervical (brachial) level cross-sections of E13.5 embryos. Scale: 200 μm (a), 50 μm (b).