Evolution of methotrexate resistance of human acute lymphoblastic leukemia cells in vitro. 
A human acute lymphoblastic T-cell line, MOLT-3, was fed with Roswell Park Memorial Institute Medium 1640 supplemented with 10% fetal bovine serum and antibiotics which contained increasing concentrations of methotrexate (MTX).  The development of drug resistance was associated initially with progressive decrease in MTX transport.  When the cells became 200-fold resistant, a rise in the dihydrofolate reductase was noted which was short-lived in the absence of the drug.  A 10,000-fold increase in MTX resistance was accompanied, in addition to further decrease in MTX transport, by a 10-fold increase in the dihydrofolate reductase activity.  While the purely transport-related resistant cell lines had a collateral sensitivity to lipid-soluble antifols, the sublines which had both transport- and enzyme-related MTX resistance contained a subpopulation highly resistant to these antifols.  Chromosome analysis of the subline with increased dihydrofolate reductase activity showed an expanded abnormally banded region in chromosome 5.