Final answer:
The specific genetic deficiency that predisposes individuals to leukemia in the presence of ionizing radiation, benzene exposure, or certain viruses is not mentioned in the given information.
Step-by-step explanation:
The genetic deficiency that can result in leukemia when combined with ionizing radiation, benzene exposure, or certain viruses is not specified in the given information. However, it is mentioned that this deficiency by itself does not cause leukemia, but the disease never occurs in the absence of this genetic factor. Therefore, the specific genetic deficiency that predisposes individuals to leukemia in the presence of these exposures is not provided.