Trimetrexate resistance of hematopoietic progenitors transduced with the MGirL22Y retroviral vector. Unsorted GFP-transduced marrow cells (dotted line), GFP-transduced marrow cells FACS sorted for high level expression of GFP (solid line), and mock-transduced marrow cells (broken line) were plated into methylcellulose cultures containing 0, 100, or 500 nmol/L trimetrexate as described in Materials and Methods. Five to eight days later, colonies were counted and percent survival, derived from the number of trimetrexate-resistant colonies divided by the number of colonies observed in the absence of drug, was determined.