 Observations on research going back well over 100 years had elucidated the two great classes of WBC - Myeloid and Lymphoid - and great advances in medicine and science have resulted from these studies.It was only natural to ask where these two great classes of cells arose, and after much work two cell types with some strong stem cell properties were isolated and defined - CMP, the common myeloid progenitor and CLP, the common lymphoid progenitor for mice.But science is an additive game and it was eventually found these progenitors were not unique, and further that the two great families of Myeloid and Lymphoid were not disjoint, but rather two partially interwoven family trees.