The amyloid precursor protein (APP) has been a focus of research into Alzheimer's disease; however, its physiological role remains unclear. Soldano and colleagues now show that ∼25% of Drosophila melanogaster lacking APP-like (Appl) — the fly homologue of APP — exhibited axonal defects in a subset of neurons within the mushroom body, a structure associated with learning and memory in flies. The WNT–planar cell polarity (PCP) signalling pathway is known to promote axonal outgrowth, and the authors found that Appl activates this pathway via activation of Abl kinase. Finally, they showed that Appl and human APP can both form complexes with PCP receptors. Thus, Appl modulates WNT–PCP signalling to regulate axonal outgrowth in the developing mushroom body.