A 4-year-old female is being evaluated for the sudden onset of multiple petechiae and bruises. She is found to have a peripheral leukocyte count of 55,000, 86% of which are small, homogeneous cells that have nuclei with immature chromatin. Indistinct nucleoli are also present. Initial tests on these immature cells are as follows: TdT, positive; PAS, positive; acid phosphatase, positive; and myeloperoxidase, negative. Based on these findings, the immature cells most likely originated from