This invention relates to antibodies or fragments thereof which interact with HER family of receptors, e.g., HER3 receptor. In particular, it relates to antibodies or fragments thereof that recognize a conformational epitope of HER3 receptor comprising residues from both domains 2 and 4 resulting in inhibition of both ligand-dependent and ligand-independent signal transduction; and allow ligand binding (e.g., neuregulin), while preventing ligand-induced activation of signal transduction. These antibodies or fragments can be used to treat a number of diseases or disorders characterized by increased levels of HER3 expression (e.g., esophageal cancer). These antibodies or fragments can be used to treat a number of diseases or disorders characterized by the antibodys or fragments ability to decrease tissue weight (e.g., prostate or uterine weights) or to induce atrophy of tissue (e.g., atrophy of male breast).