ErbB2 protein expression is heterogeneous (i.e. some parts of the tumor produce ErbB2 and some do not) in a significant fraction of ErbB2-positive breast cancers that display a relatively homogeneous amplification of the ErbB2 gene (the latter is normally detected by Fluorescence In Situ Hybridization (FISH)) [52]. This evidence concerns the gene ERBB2 and breast cancer.