The earlier 1999 phase I study reported by Seibel et al. [61] explored the safety and activity of B43-PAP at two different dose levels (1 and 1.5 mg/m2/day given daily on days 9–13 and 21–25) given concomitantly during reinduction with standard VPLD chemotherapy to 24 children with relapsed CD19+ ALL. Here, CD19 is linked to acute lymphoblastic leukemia.