Light can be used as a precise and reversible trigger for the activation of optogenetic tools with subcellular resolution. The interaction of the photoreceptor PAL and aptamer 53 was integrated into a CRISPR/dCas9 system, which can be applied for light-controlled activation of gene expression. Here, we describe a protocol for in vitro application of light-dependent overexpression using eBFP as a proof of concept. The experiment can be done in 3 days, which is split into cell seeding, transfection, and evaluation by flow cytometry. The method is broadly applicable including the upregulation of endogenous genes.