Concordance among three methods of RFC mRNA quantification. (A) Comparison of Northern blotting and competitive RT-PCR using stRNA as an external standard. (B) Comparison of Northern blotting and RT-PCR using β-actin as an internal standard. (C) Comparison of competitive RT-PCR using stRNA and RT-PCR using β-actin. The amount of RFC mRNA from the CCRF-CEM/wt cell line was assigned a value of 100%. The RFC mRNA amount from other cell lines was calculated as a percentage of CCRF-CEM/wt. CCRF-CEM/wt (⧫), CEM/T (◥), CEM/MTX (░), Nalm6 (▪), K562/wt (◍), K500E/MTX (•).