Figure 1.

Apc heterozygosity index for adenomas in B6 Min/+ and AKR Min/+ animals. The Apc heterozygosity index, Het_Apc, was defined by the Apc⁺/Apc^Min band intensity ratio for tumor DNA, normalized to that ratio for DNA from adjacent normal tissue. For 59 tumors arising spontaneously in B6 Min/+ animals (filled bars), the values of Het_Apc are distributed approximately as Gaussian with mean 0.32 and SD 0.17. For 64 tumors (22 spontaneous and 42 arising after ENU treatment) in AKR Min/+ animals (empty bars), the distribution of Het_Apc differs strongly from that of B6 Min/+ tumors (P = 3.5 × 10⁻⁹ by the Wilcoxon rank sum test). The AKR Min/+ indices are well approximated by a mixture of two Gaussian distributions, with 53% of the tumors centered around 0.32 (the LOH class) and 47% of the tumors around 0.95 as determined by the method of maximum likelihood. With a definition of maintenance of heterozygosity as Het_Apc ≥ 0.64, the probability of misclassifying a tumor with LOH is only 3%. No significant differences in the distribution of Het_Apc for tumors from AKR Min/+ mice were observed as a function of ENU treatment or method of tumor dissection and DNA isolation.