Fig. 9. Schematic diagram summarizing the transcriptional cascade that controls the development of laminae I-II circuitry. In this cascade, we hypothesize that an unidentified transcription factor X, cooperating with Lbx1, activates and maintains Lmx1b expression, which in turn maintains Rnx expression. Lmx1b most probably activates Drg11 expression directly, whereas Rnx helps to maintain Drg11 expression. Rnx is also required for maintaining Lmx1b expression. Lmx1b acts in part through Rnx and Drg11, and in part through other unknown genes to repress Zic1 and Zic4 expression (not shown). Lmx1b and Rnx maintain Ebf1 and Ebf3 expression in laminae I-II neurons (not shown). Together these transcription factors coordinate the migration and differentiation of the superficial dorsal horn neurons, and subsequent innervation of TrkA⁺ afferents.