Two morphogens are produced along the anterior-posterior axis of the Drosophila embryo.

Morphogen 1 is expressed most highly at the anterior pole, tapering with distance from its source.

Morphogen 2 is expressed highly at the posterior pole, also tapering with distance from its source.

Expression of Gene 3 is stimulated only by intermediate levels of morphogen 1.

Expression of Gene 4 is inhibited by any expression of gene 3 and morphogen 2.

Expression of Gene 5 is stimulated only by intermediate and high levels of morphogen 1.

Expression of morphogen 2 is inhibited by any level of gene 5 expression.

In the figure above, the arrow denotes the expression of which gene or morphogen?