SUMO1 and infection: Our findings indicate thatoverexpression of SUMO-1 in 293T and TE671 cells enhances an intrinsic block toN-MLV infection of human cell lines (Figure 1C and 2C).We show that this enhanced block is dependent on virus tropism (Figure 3A), occurs before reverse transcription(Figure 3C), and can beabrogated by pre-infection with restricted virus (Figure 3B).