The activity of traditional small-molecule inhibitors of microtubules cannot be restricted to target cells in research and chemotherapy. Borowiak et al. have developed optically controlled photostatins against microtubules by chemically modifying the microtubule inhibitor combretastatin A4 so that it can be reversibly switched between its trans, inactive form and its cis, toxic form in response to light; a series of photostatins were generated. The toxicity of photostatins is highly specific and reversible; photostatins were non-toxic to cancer cell lines in the dark but highly toxic in the light (maximum toxicity was achieved with 390 nm light). Photostatins were shown to inhibit tubulin polymerization, and their biological importance was demonstrated on several levels. Photostatins induced cell death and mitotic arrest in several cell lines. Moreover, they reversibly controlled microtubule dynamics and induced mitotic arrest in a single cell within a developing Caenorhabditis elegans embryo.