BACKGROUND AND OBJECTIVE: CD10, initially known as cALLA, was identified as one of the earliest markers expressed by leukemic cells of the lymphoblastic lineage. This review summarizes what has been discovered about this ubiquitous molecule since anti-cALLA antibodies allowed its detection on leukemic cells. It also attempts to specify the selectivity of CD10 in acute leukemias as a subclassification or prognostic tool. EVIDENCE AND INFORMATION SOURCES: The material used for this review includes articles identified as relevant through a meta-analysis in the Medline database, as well as personal publications or data issued within the French Study Group on the Immunology of Leukemias (GEIL). STATE OF ART: CD10 now stands as much more than a mere leukemic marker. It belongs to a rather large family of exopeptidases expressed in a variety of tissues and mostly involved in the activation or deactivation of peptides through the removal of terminal amino acids. CD10 expression on leukemic cells remains a useful subclassification tool for B-lineage leukemias, but it can also be found on other types of leukemic cells. PERSPECTIVES: Much remains to be discovered regarding the physiological role of CD10 during the maturation of normal B-lineage lymphocytes and about its functions-or lack of activity-on leukemic cells. It could also provide a valuable tool for further dissecting B-lineage leukemias when the quantitative aspect of its expression on blast cells is taken into account.