HEY1 and breast cancer: Data suggests that deregulation of Notch signalling is an early event in breast cancer tumorigenesis, with accumulation of NICD and increased Hey1 expression detected in a broad range of subtypes, including ductal carcinoma in situ and epithelial hyperplasia (Stylianou et al., 2006; Mittal et al., 2009; Zardawi et al., 2010).