The past year has seen major advances in our understanding of the reco
mbination mechanism by which antibody and T cell receptor genes are as
sembled during lymphoid development. The initial cleavage events can b
e carried out in vitro by purified RAG1 and RAG2 protein. In addition,
a number of genes involved in later steps of the reaction have been c
loned, opening the way for an in-depth biochemical analysis of this cr
itical developmental process.