UASt regulatory sequences drive expression of a photoactivatable, dominant-negative form of Cdc42 (carries a T17N amino acid substitution) that is tagged at the N-terminal end with mCherry. The Cdc42 protein has been rendered photoactivatable by being tagged with Tag:AsLOV2; in response to blue light illumination, the conformation of the Tag:AsLOV2 is altered, releasing the active site of the dominant-negative Cdc42 protein.