(color online) (a) HER2 gene expression levels in MCF-7 and SKBR-3cells by multivolume digital PCR. The normalized ratio of HER2 to GAPDH in gene expression is represented by the red diamonds. (b) Ratios of HER2 gene expression between SKBR-3 and MCF-7 cells measured by the present multivolume digital PCR and conventional quantitative real-time PCR, respectively (from Ref. [17], Fig. 5).