The histogram of Figure 1.17 shows the frequency with which mutations are found at each base pair in the lacI gene of E. coli. The statistical probability that more than one mutation occurs at a particular site is given by random-hit kinetics (as seen in the Poisson distribution). So some sites will gain one, two, or three mutations, while others will not gain any. But some sites gain far more than the number of mutations expected from a random distribution; they may have 10กั or even 100กั more mutations than predicted by random hits. These sites are called hotspots. Spontaneous mutations may occur at hotspots; and different mutagens may have different hotspots.