This application concentrates upon determining the rates and spectra of mutation arising during a single cycle of HIV and MLV vector viral replication.One objective is to develop HIV-based packaging cells needed to carry out HIV mutation rate studies. Once such HIV-based packaging cells are available, they will be utilized for HIV forward mutation rate (loss-of- function) analysis. One of the genes to be employed as a reporter is the lacZa gene. The studies to be done with MLV are an extension of already described work indicating that MLV vectors can undergo genetic rearrangement at a particularly high rate, including a high rate of transduction of sequences from infected cells. Experiments are proposed to extend MLV mutation rate analysis to a number of genes to address the sequence-dependence of mutational events.