Comparison of p15 and BPI mRNA levels in density-fractionated rabbit bone marrow cells. mRNAs from Percoll density gradient fractions of increasing density (1-5) were analyzed by RT-PCR as described in Materials and Methods. Absence of contamination (-cDNA control) was verified by subjecting a sample of the cDNA diluent (water) to PCR under each condition. Results shown are representative of two similar experiments.