PRL and breast cancer: Evidence from preclinical and clinical data show that elevated prolactin levels cause proliferation of breast tissue and result in breast enlargement, which may be markers for an increased risk of breast cancer development and important factors in the carcinogenicity of mammary tissue (Ingram et al. 1990; Arendt et al. 2011; Harvey et al. 2006; Clevenger et al. 2009; Carver et al. 2009).