To elucidate the mechanism of VH to VHDJH joining (VH replacement), we have assayed a pre-B cell line for ability to perform V(D)J recombinase-mediated rearrangement events between an unrearranged VH gene segment and a VHDJH rearrangement within a retroviral recombination substrate. V(D)J recombinase-mediated inversional joins between the VH gene segment and the VHDJH rearrangement within this substrate activate a drug-resistance gene allowing selection of cells that performed the event. We have used this method to identify and isolate VH to VHDJH joins within the introduced substrate. Analyses of the resultant signal and coding joins of such rearrangements demonstrated that VH replacement recombination is mediated by a heptamer-like sequence embedded in the 3' region of the VH segment in the assembled VHDJH join and the heptamer-spacer-nonamer recognition signal of the unrearranged VH segment.