IK mRNA expression in isolated CD34⁺ cells. RT-PCR products were analyzed by 5% polyacrylamide gel electrophoresis and autoradiographed. Positive controls (100 and 10 ng total RNA from IK-positive RJ cells) and a negative control (without RNA) were included to ensure that all tested samples were in the linear region of the PCR curve. GAPDH for each sample was amplified as an internal control. Signals were found at the expected sizes (692 bp for IK and 240 bp for GAPDH), indicating that the PCR was specific. (A) IK expression from LD mononuclear cells and 2 CD34⁺ cell samples purified from cord blood (CB). Scanning densitometry of the IK mRNA signal relative to the GAPDH mRNA signal shows the following values: lane 2 (RJ cells, 10 ng RNA), 24.6%; lanes 4 and 5 (CB CD34⁺ cells), 6.0% and 13.1%, respectively; lane 6 (CB mononuclear cells), 46.7%. (B) IK expression in total CD34⁺ cells purified from bone marrow and in 2 different subsets of medullar CD34⁺ cells: CD34⁺DR^low and CD34⁺DR^high cells.