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The anaplastic large cell, CD30 positive lymphomas represent a heterogeneous group of lymphomas in which immunocytochemical and molecular investigations have demonstrated the existence of malignancies of T, B or undetermined origin. The recent identification, in a few cases of this group of a chromosomal 5q35 breakpoint may allow the individualization of a peculiar disease entity. In these cases, the 5q35bp has been found to be a permanent abnormality, present in five human permanent cell lines and associated with various translocations including t(2;5), t(1;5) and t(5;6). A primitive myelo-monocytic origin of these 5q35bp cell is suggested on the basis of the following arguments: they express the c-fms proto-oncogene which encodes the macrophage growth receptor (CSF-1-R) the c-fms is mapped on 5q33,34 close to the 5q35bp the express spontaneously CD68 the treatment by phorbol-diester of a 5q35bp cell line (DEL cell line) induces an immunodependent phagocytosis and a modulation of expression of c-fms, CSF-1 and TNF alpha. Because some 5q35bp cell lines also presents rearrangements for TCR-bêta, or IgF, these data suggest an ancestral stem cell origin, prior to the T, B and myelomonocytic differentiation. Whatever its origin, the 5q35bp abnormality has been mainly encountered in children's malignancies. It has been constantly associated with clinical and biological manifestations of a condition recognized by paediatricians as malignant histiocytosis. For this reason, the 5q35bp may today represent the best criterion for the identification of malignant histiocytosis in childhood.