A DC motor control apparatus includes Hall effect devices each generating a pulse each time a DC motor rotates a predetermined angle. The method may be used to screen either a chemical library in order to identify a molecule having a gene-specific lethal property in the cells, or to screen a group of DNA molecules in order to identify among them one or more modulators of gene function which are synergistically lethal to the cells together with an incapacitated gene of interest. The nonvolatile memories have a write-protect function.