At 2-weeks post-Dox administration (3-weeks post- transplantation), hFVIII:Ag levels increased to 13–43% of normal human plasma levels (Fig. 2Ai). However, masses had formed in the abdomens of all mice. Laparotomies revealed spleen tumors, which were histologically confirmed as teratomas, consisting of muscle cells, chondrocytes, glial-like cells, and hair-root cells (Fig. 2Aii). In one mouse that underwent a splenectomy, hFVIII:Ag became unde- tectable within 3-weeks post-surgery (6-weeks post- transplantation). Reverse transcription-PCR analysis showed detectable human F8 mRNA only in spleens with teratomas, but not in the livers (Fig. 2Aiii), indicating that plasma hFVIII: Ag was derived primarily from spleens with teratomas.