Contrary to current ‘CD44+CD24-/low centric’ thought regarding the developmental plasticity of tumor-initiating mammary epithelial cells [6,48], recent studies have revealed that not only CD44+CD24-/low cells can give rise to CD44+CD24+ cells, as expected for a cancer stem cell [49], but that the converse can also occur; CD44+CD24+ cells can give rise to their CD44+CD24-/low counterparts and single cells from either phenotype are capable of initiating tumors as xenografts with high efficiency [50]. The gene discussed is CD44; the disease is cancer.