STAT5 is involved in soft agar growth of K562 cells. K562 cells were electroporated with STAT5δ750 (B and C), the empty expression vector (LNCX, A and D), or STAT3β (C and D), after which the cells were selected on G418 (1 mg/mL) for 10 days. Cells were then plated in soft agar. Seven to 10 days later, colonies were counted as described in Materials and Methods. STAT5δ750 partially inhibits the capacity of K562 cells to grow in soft agar, while STAT3β did not have an effect.