Pentasomy 21 characterizing spontaneously regressing congenital acute leukemia

Pentasomy 21 was found to characterize the proliferating cells in a case of transient congenital acute leukemia (or congenital acute leukemia) with spontaneous remission. The patient was phenotypically normal, and cytogenetically no evidence could be found for the existence of a mosaic with a normal cell line and one with more than two No 21 chromosomes. The importance of these findings is discussed.