RT-PCR analysis of Mpl receptor mRNA. RT-PCR was performed to detect the presence of c-mpl or GAPDH mRNA in HEL cells (lanes 4 and 5), K562 cells (lanes 6 and 7), CD34⁺ human marrow cells (lanes 8 and 9), and colonies derived from day-5 (lanes 10 and 11), day-7 (lanes 12 and 13), and day-10 (lanes 14 and 15) BFU-E progeny. The primers used for amplification were specific for the extracellular domain of Mpl (lanes 3, 4, 6, 8, 10, 12, and 14) or for GAPDH (lanes 2, 5, 7, 9, 11, 13, and 15). Cloned human mpl cDNA was used as a positive control (lane 3). Negative controls were performed using no input cDNA (lanes 1 and 2).