tokens: Also at 6 weeks , rats given one , two , or four doses of N-OH-AAF had lacI mutant frequencies in the liver of 97.6 , 155.6 , and 406.8 x 10 ( -6 ) , respectively , compared with a control frequency of 25.7 x 10 ( -6 ) ; rats given four doses had lacI mutant frequencies in spleen lymphocytes of 55.8 x 10 ( -6 ) compared with a control frequency of 20.4 x 10 ( -6 ) .