We designed and validated a novel single-reaction multiplex droplet digital PCR (ddPCR) assay to quantify two alleles of the single nucleotide polymorphism (SNP) rs1058808 within ERBB2 simultaneously with two reference control genomic regions, the chromosome 17 centromere (CEP17) and a breast cancer CN stable region located near cytoband 2p13.1 (CNS-2p13.1). The gene discussed is ERBB2; the disease is breast cancer.