Primer extension of RNA samples from primary and secondary recipients. Mice from experiment 1 were killed and their marrow cells used to reconstitute lethally irradiated secondary recipients. Each successive group of 4 lanes represents RNA samples from a primary mouse BM, spleen, and peripheral blood of two secondary recipients. Lanes 1 through 4, samples originating from mouse 1; lanes 5 through 8, mouse 2; lanes 9 through 12, mouse 3; and lanes 13 through 16, mouse 4. Lane 17 represents RNA from a mouse transgenic for the human β-globin gene. The arrows at the left of the figure indicate the predicted positions for human β-globin (Hum β) and mouse β-globin (M β^maj).