Surprisingly, it has not been clearly established whether the co-expression of two NRG1 receptors in these neoplasms promotes tumorigenesis and, if they do, whether they do so by increasing receptor numbers (thereby rendering tumor cells more responsive to NRG1 or other erbB ligands) or whether these two receptors instead make distinct, critically important contributions to tumor initiation and progression. Here, NRG1 is linked to neoplasm.