Functional genetic screens on mutant backgrounds have been successfully used in lower organisms to investigate biological processes. However, few identical screens have been performed in mice. Recombinase activating gene-1 deficient (Rag1−/−) mice have a severe T-cell developmental block owing to lack of rearrangement of their T-cell receptor (TCR) genes. Using a retroviral cDNA library derived from wild-type embryonic thymocytes we performed a suppressor screen in Rag1−/− hematopoietic cells and recovered TCRβ. This is the first demonstration that targeted genetic screens are feasible using transduced primary cells in vivo. Consequently, this technique can be used to interrogate multiple blood lineages using diverse hematopoietic mouse mutants.