CD28 and leukemia: Several leukemia-related specific pathways [43–45], including co-stimulation by the CD28 family, HSC and multipotent progenitors, targets of MLL-AF9 fusion pathway, AML prognosis, TCR pathway, P38 and MK2 pathway, and FLT3 signaling, which were demonstrated to play crucial roles in the regulation of leukemogenesis and progression.