Single-colour CISH has previously been documented to be a useful alternative to FISH, and in several studies the concordance levels of HER2 status between single-colour CISH and FISH were found to be in the range of 91–100%.1,2,7,19–26 However, a single-colour CISH system is limited, as only one type of signal can be evaluated on the section, and HER2 breast cancer specimens that do not have a clear non-amplified or highly amplified HER2 status should be retested on a serial section for a possible chromosome 17 polysomy. This evidence concerns the gene ERBB2 and breast cancer.