One of the proteins which participates in hematopoiesis, PEBP2/CBF, is also associated with osteogenesis in mammals. Embryologically, two types of bone can be distinguished. Intramembraneous bone is formed directly by bone cells within connective tissue. This type of bone gives rise to the skull and face. The rest of the skeleton originates by replacement of cartilage through a process known as endochondral ossification. This second type of bone is derived from multipotent cells that are capable of differentiating into muscle, dermis and cartilage. It has recently been discovered that one of the key proteins which are responsible for the specialization of multipotent cells leading to bone belong to the family of Runt-domain proteins. The family member associated with osteogenesis, known as CBFa1 or OSF2, contains the same DNA-binding domain as that found in the hematopoietic transcription factor PEBP2/CBF.