Nucleotide sequencing gel of individual cDNA clones of the nucleotide sequence in the region of RFC1 codon 25. The sequencing gel was arranged so that all dideoxy termination reactions for the different templates were grouped together, which facilitated recognition of aberrant sequences. cDNAs included in the gel were from WT ZR-75 human breast cancer cell line, a transport-deficient MTX-resistant ZR-75 cell line,8 WT MOLT-3, MOLT-3/MTX₂₀₀, and MOLT-3/MTX_10,000 cells. Arrow designates site of mutation in the MOLT-3/MTX_10,000 cDNA clone.