Predicting the toxicity of drugs that affect splicing. Four genes with cis-regulatory elements depicted as in Figure 1 are on the right, with the corresponding splicing regulators symbolized as circles. Chemical compounds are illustrated as cylinders on the left and are connected to the regulator(s) with which they interfere. The most toxic drugs would be those that affect a common regulator that binds to elements common to many genes, for example, drug C affecting regulator V. Drugs that affect several regulators with uncommon target elements would be less toxic: for example, drug B interfering with regulators I, II, III, and IV. The least toxic class would be drugs that specifically affect an uncommon regulator, such as drug A affecting regulator III.