A calculation of the probability of X-ray or gamma-ray leukemogenesis per cell in humans is made. It is assumed that the theory is of the two-hit type. The first hit damages DNA repair capability of base damage. The second hit causes derepression of a 2 repressor system. Sequential repression of gene expression occurs. Inactivation of the first repressor permits synthesis of the second which will repress the expression of the leukocyte maturation gene. The first repressor is inactivated by X-ray damage to its corresponding operator region. I have used a more realistic value of immune efficiency for the bone marrow and I have considered spontaneous leukemogenesis. Agreement with the best algebraic fit of Kellerer and Rossi for the Nagasaki victims is good.