Schematic diagram of the retroviral vectors used in gene transfer. Retroviral vectors N2 and AsADA are Moloney murine leukemia virus–based vectors. N2 contains the neo gene, expressed from the viral LTR promoter. AsADA contains neo, expressed from the viral LTR promoter, and the human ADA gene, expressed from its endogenous promoter, As. Relevant restriction enzyme sites and the distance between them are indicated. ψ⁺ represents the packaging signal. The 930-bp EagI-AvaI fragment in the neo coding region was used as a probe in experiments described in Figs 2 and 3.