Fig. 3. FS colonization by B committed precursors at E13. (A) CD45⁺ Ter119^- cells from E13.5 FS and FL were sorted and analyzed by RT-PCR for the expression of transcription factors involved in B cell commitment: E47, Ebf and Pax5. E15 FL cells depleted of erythrocytes were used as a positive control. (B) FS and FL from E13.5 Rag2^GFP transgenic embryos were scored by FACS for B220 and CD43. We also obtained B220⁺ CD43⁺ B progenitors that express GFP.