MYC and breast cancer: While transcriptional regulators such as USF/c-myc [7], NFkB [8] and p53 [9] have been reported to contribute to the regulation of CXCR4 expression, they are ubiquitously expressed; estrogen receptor-dependent up-regulation of CXCR4 in breast cancer cells has also been reported [10], but it does not account for the fact that a high level expression of CXCR4 predicts a poor prognosis for a “triple-negative” type of breast cancer, which does not express a hormone receptor [3].