In agreement, HPK exhibits considerable binding to an immobilized peptide containing the extracellular domain of human HER3, which is blocked by preadsorption with the peptide in vitro (Figure 2A) and on HER3-expressing breast tumor cells (Figure 2B; cell surface HER3 levels shown in Supplementary Figure S2), confirming the receptor specificity of the ligand. The gene discussed is ERBB3; the disease is breast neoplasm.