Abstract: Normal human CD4+ T-lymphocytes can undergo malignant transformation during prolong cultivation in conditions of high endonuclease G (EndoG) expression or after DNA damage. The aim of this work was to study biochemical and cytogenetic features of transformed ex vivo human malignant CD4+ T- lymphocytes, as well as biochemical and morphological characteristics of tumors that develop in athymic mice after transplantation of these cells. The telomerase activity was higher and telomere length was shorter in tumor cells than in control cells. Transformed malignant cells exhibited a high level of chromosomal aberrations. Expression of genes regulating the cell cycle changed in transformed malignant CD4+ T-lymphocytes and tumor cells. The tumors that developed were classified as multicomponent Т-cell lymphomas and panniculitis-like T-cell lymphomas. Thus, transformed CD4+ T-lymphocytes can generate malignant tumors of various histogenesis. © 2019, Pleiades Publishing, Ltd.