Anaplasia refers to unifferentiated cell growth in a malignant neoplasms (cancers). Lack of differentiation is considered a hallmark of malignancy. The term anaplasia literally means "to form backward." It implies dedifferentiation, or loss of structural and functional differentiation of normal cells. It is now known, however, that at least some cancers arise from stem cells in tissues; in these tumors failure of differentiation, rather than dedifferentiation of specialized cells, account for undifferentiated tumors.