Fig. 10. (A) Schematic summary of regulatory functions of PDF2 (blue) and ATML1 (red) in L1 layer-specific gene expression. The illustration incorporates potential formation of homo- and heterodimers. Red arrows indicate transcriptional activation mediated by the interaction of PDF2 and/or ATML1 with the L1 box located upstream of each gene. (B) A hypothetical model in which the L1 layer is established and maintained in the embryo. Continuation of an autoregulatory loop for the PDF2 and ATML1 expressions is maintained in the protodermal layer, and an as-yet-unidentified suppressor(s) functions (orange) in switching off the loop in the basal and inner cells of the 16-cell embryo.