ERBB2 and neoplasm: Following this, the 2013 and 2018 ASCO/CAP guidelines defined intratumoral heterogeneity as the presence of a second population of tumor cells, of which 10% or more were tumor cells have a different HER2 copy number and/or HER2/CEP17 ratio; the guidelines recommended additional counting and reporting of at least 20 cells in the second tumor population [5,34].