Control reactions for RT dependence. Reactions for RT of RNA from human platelets or human megakaryocytic cell lines were performed as described in the Materials and Methods section either in the absence (-) or presence (+) of RT. The reaction products were then subject to PCR amplification using the oligonucleotide primer pair specific for exons 3-6 of human factor XI. PCR products were obtained only for reactions in which RT was present. Lanes 1, platelet RNA; 2, HEL 92.1.7 cells; 3, MEG-01 cells; 4, CHRF-288-11 cells; 5, 293 cells transfected with wild-type FXI cDNA. The position of the molecular weight marker is shown at the left of the figure.