HOXA5 upregulates both p53 promoter-reporter constructs and endogenous p53 synthesis, leading to apoptosis. HOXA5 is detectable in approximately one-third of primary tumors. Lack of HOXA5 expression strongly correlates with methylation of its promoter region, suggesting a causal role for methylation in the silencing of HOXA5 gene expression. HOXA5 expression is an important step in tumorigenesis and loss of expression of p53 in human breast cancer may be primarily due to lack of expression of HOXA5.