The Philadelphia (Ph) chromosome was the first chromosomal abnormality associated with a specific leukemia, chronic myeloid leukemia (CML). This chromosome arises from the t(9;22)(q34;q11) translocation which results in the juxtaposition of the bcr gene and the abl proto-oncogene. This BCR/ABL fusion gene encodes for a hybrid protein with the capacity of oncogenic transformation of hematopoietic cells. Nonetheless, very few myeloproliferative disorders (about 10%) included under the generic term of CML have no Ph chromosome. Half of these Ph-negative CML have the BCR/ABL fusion gene (BCR-positive) and are considered equivalent to Ph-positive CML. In contrast, the patients without detectable BCR/ABL fusion (BCR-negative) fulfil the criteria for atypical CML (aCML) of the French-American-British (FAB) classification, despite considerable variability at the individual level. Due to the very small number of patients with precise cytological descriptions already published, cooperative studies focused on aCML are warranted to draw definitive conclusions and to provide some pointers on physiopathology.