The present application discloses a lentiviral transfer system which includes : (i) a self-inactivating transfer vector comprising : multiple gene units, wherein each gene unit includes a heterologous nucleic acid sequence operably linked to a regulatory nucleic acid sequence; and (ii) a helper construct which lacks a 5′ LTR, wherein the 5′ LTR has been replaced with a heterologous promoter, in which the helper construct further comprises : a lentiviral env nucleic acid sequence containing a deletion, wherein the deleted env nucleic acid sequence does not produce functional env protein; and a packaging signal contains a deletion, wherein the deleted packaging signal is nonfunctional.