THE imaginal discs of Drosophila, which give rise to the adult appendages, are patterned during a period of intense cell proliferation. The specification of differing regions occurs in some cases by subdividing the disc epithelium into lineage compartments(1). However, in most cases precise boundaries are formed between different cell types without early compartmentalization(2). One such boundary occurs between the wingless (wg)-expressing cells of the wing margin and the adjacent proneural cells, which give rise to margin sensory bristles. Here we show that this boundary arises in part by a mechanism of ’self-refinement’, by which wingless protein (Wg) represses wg expression in adjacent cells. Cells unable to receive the Wg signal do not resolve the boundary between wg-expressing and proneural cells.