TALE4::VP64 is an artificial transcriptional activator ('driver') that is composed of a synthetic transcription activator-like effector (TALE) DNA-binding sequence fused to the VP64 transcription activation domain. The TALE repeat sequence of TALE4::VP64 has been designed to specifically bind the 20bp VAS4 sequence. VAS4 and TALE4::VP64 thus form a binary expression system that can be used to control the spatial and temporal expression of a gene of interest: a transgene or modified endogenous locus carrying the target gene of interest downstream of VAS4 sequences is combined with a second transgene or modified endogenous locus encoding the TALE4::VP64 driver expressed in the required expression pattern (FBrf0237269).