ERBB4 and breast cancer: However, in the case of HER4, the few clinical studies carried out so far have suggested that its expression is associated with favourable prognosis (Thybusch-Bernhardt et al, 2001; Suo et al, 2002) and recently it has been reported that HER4 mediates ligand-dependent antiproliferative and differentiation responses in human breast cancer cells (Earp et al, 2003).