WITH THIS ISSUE OF BLOOD we launch a new feature, Controversies in Hematology. The series is designed to identify both long-standing and relatively recent topics in Hematology and Oncology on which considerably diverging opinions exist, and to highlight the critical data supporting each view. The format is one of point and counterpoint. Each discussant will have an opportunity to put forth the critical arguments supporting their position. The papers will be exchanged and each author then allowed to rebut the arguments of the opposing discussant. We feel that this format will allow the readership to quickly appreciate the question being addressed and the nature of the evidence in support of each view. The reader is then left to his or her own conclusions. We also hope each Controversy will evoke discussion and stimulate additional investigation in the field.

The inaugural chapter in the series asks what are the mechanisms responsible for commitment of multipotent hematopoietic progenitors to cells of each blood lineage. Evidence from many quarters indicate that growth factors are essential for hematopoietic development. However, the precise role played by these proteins, whether to direct multipotent cells down a particular cellular pathway or to merely support the survival of cells that have intrinsically selected a specific hematopoietic lineage, remains controversial.