Naphthol phosphate blocks silver background staining resulting from the HER2 & CEN17 BISH procedure. Images show HER2 gene-protein staining results obtained without (A, B) or with naphthol phosphate (C, D) on formalin-fixed, paraffin-embedded (FFPE) HER2-negative MCF7 (A, C) and HER2-positive Calu-3 (B, D) xenograft tumors. In the absence of naphthol phosphate, non-specific silver deposition from the HER2 BISH detection procedure obscures the BISH signals for the HER2 gene and CEN17 targets (A, B) whereas the use of a BISH hybridization buffer containing naphthol phosphate eliminates the non-specific silver deposition (C, D). Some silver deposition was also seen in DAB staining (A). In the absence of naphthol phosphate, non-specific silver deposition occurred in mouse cells (yellow asterisks) (A, B) and mouse cells were confirmed without HER2 and CEN17 BISH signals by using naphthol phosphate (C, D). 60×.