Measurement of lacI mRNA in splenocytes. RNA was extracted from splenocytes harvested from BB⁺ and BB⁻ mice and used to make cDNA with random hexamers. PCR amplifications for GAPDH and for lacI were performed on 5-fold serial dilutions of cDNA. Genomic DNA from a BB⁺ mouse was used as a positive control in the GAPDH and lacI PCR. M indicates Mock; and RT, reverse transcription.