Rescue of blast and hematopoietic colonies from Scl^-/- FLK-1⁺ precursors. (A) Kinetics of FLK-1 expression in Scl⁺/⁺ and Scl^-/- EBs. Numbers above the graphs indicate the day of differentiation. Percentages of FLK-1⁺ cells are indicated above the gates. (B) Blast colony potential of day 4 Scl^-/- FLK-1⁺ EB-derived cells following 24 hours of coculture with either SCL or control (MSCV) virus-producing cells. Colony numbers are presented per 10⁵ FLK-1⁺ cells seeded onto virus-producing cells. Data are presented as mean number of blast colonies from 3 dishes. Bars, where visible, represent SEM. (C) Hematopoietic progenitor potential of the infected populations described in panel B. □ indicates ERY/P; ▪, MAC; and ▦, MIX. Data are presented as mean number of colonies from 3 dishes. Bars, where visible, represent SEM. (D) Expression of the endogenous and viral Scl transcripts in day 3.5 Scl^+/+ FLK-1⁺ (sort, +/+), day 4 Scl^-/- FLK-1⁺ (sort, -/-), and 2 pools of rescued Scl^-/- FLK-1⁺ cells (rescue, -/-) harvested following 3 days of coculture with SCL virus-producing cells (SCLVPCs) and 5 days of expansion in KL, IL-3, and EPO. To facilitate the detection of endogenous Scl, primers were designed against the 3′ UTR of the Scl gene. Detection of viral Scl was carried out using a 5′ primer designed against the 3′ translated region of the Scl gene and a 3′ primer designed against the PGK promoter present only in the retroviral transcript. (E) Expression analyses of the populations described in panel D were performed using probes designed against the 3′ UTR of the indicated genes.