Use of alternative control probes from these regions to assess HER2 by FISH in HER2-equivocal as well as HER2-not-amplified breast cancers resulted in high rates of false-positive ratios (HER2-to-alternative control ratio ≥2.0) owing to heterozygous deletions of control p-arm genomic sites used in ratio denominators. The gene discussed is ERBB2; the disease is breast carcinoma.