ERBB2 and neoplasm: In 11 (61%) out of these 18 patients, the discrepancy was due to pathologist error in scoring of HER2 protein expression or of HER2 gene status, in 5 (28%) cases, protein or genetic tumor heterogeneity was considered to be the source of the disagreement between IHC and FISH, and, in 2 (11%) patients, no reasons for the discrepancy were found, as the FISH and IHC analyses were truly discordant.