ERBB2 and cancer: The HER2/CEP17 ratio obtained by digital PCR [r] was expressed as follows, based on the assumption that gene copy numbers of cancer cells are homogeneous and that non-cancerous cells are genetically stable with diploid chromosomes (Fig 1A and 1B):r=Bx+2(1−x)Ax+2(1−x)(1)where [r] is the ratio of HER2 to CEP17 obtained by digital PCR (0 < r); [x] is TCR (0 ≤ x ≤ 1); [A] is CEP17 copy number in a single cancer cell (0 ≤ A); and [B] is HER2 copy number in a single cancer cell (0 ≤ B).