Mouse Six1 enhancer for dorsal root ganglion neurons directs expression in Rohon-Beard cells earlier than Xenopus enhancer. (A) Xenopus enhancer directed the expression of enhanced GFP (EGFP; green) in Rohon-Beard (RB) cells (magenta, labeled by Isl1/2) not at Stages (St.) 25/26, but at St. 41/42, and in dorsal root ganglion (DRG) neurons at St. 49. (B) Mouse enhancer directed expression of EGFP in RB cells at St. 25/26 and 41/42, and in DRG neurons at St. 49. For A and B, bar graphs show the percentage of EGFP-positive RB cells in 250 μm of the thoracic level. Data are mean ± standard error of the mean. Arrowheads: EGFP- and Isl1/2-double-positive RB cells; dashed lines demarcate the position of the spinal cord (sc); so, somite; drg, dorsal root ganglia. Scale bars: 25 μm. (C) Schematic representation of timing of Six1 expression in Xenopus and mouse. Xenopus six1 enhancer for sensory neurons (xSix1-8) directs the expression of Six1 (red arrow) to alter the sensory system from RB cells (pink line) to DRG neurons (blue line). In mouse, Six1 enhancer (mSix1-8) mediates SIX1 expression earlier during trunk sensory development. This altered timing of SIX1 expression inhibits the development of intramedullary sensory cells and promotes DRG neurogenesis.