Abnormal expression of homeobox genes leading to loss of differentiation is observed in breast cancer where expression of Hoxb6, Hoxb7, Hoxc6, Hoxc8, Hoxd4, Hoxd8, Hoxd9, Hoxd10, Hoxa5 is lost in mouse mammary tumours and Hoxa1, Hoxd3, Hoxd12, Msx1, Six1 and Oct3 are instead expressed in cancer, but absent or lowly expressed in the normal differentiated gland [8]. This evidence concerns the gene MSX1 and breast cancer.