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The duct is specified by Scr, Exd and Hth in combination with EGF signaling. EGF signaling blocks expression of Fkh in the most ventral cells of PS2, allowing the expression of duct-specific genes such as Trh and its downstream targets, btl, eyg and, presumably, other genes. The absence of Fkh expression in the ventral cells also allows expression of two other duct-specific factors: Dri and Ser. Ser in duct cells is important for establishing the imaginal ring cell fate.
