The V(D)J recombinase enzyme complex is responsible for the development of a diverse immune system by catalyzing intra-molecular rearrangements of immunoglobulin (Ig) and T cell receptor (TCR) genes at specific recombination signal sequences (RSSs). This enzyme complex has also been implicated in mediating pathologic and non-pathologic intra- and inter-molecular genomic rearrangements at cryptic (ψ) RSSs outside the immune system loci in lymphoid cells. We describe here two V(D)J recombinase mediated genomic rearrangements resulting in alterations at theHPRTlocus in human T-cells. These are inter-chromosomal insertions in which DNA fragments are inserted at breakpoints generated by V(D)J recombinase cleavage at ψ RSS sites in theHPRTlocus at Xq26. In the first, a TCR α signal ended segment from chromosome 14q11 is inserted at a ψ RSS in intron 1 of theHPRTlocus. In the second, a DNA fragment from 9q22 is integrated between the coding ends generated by a V(D)J recombinase mediatedHPRTdeletion. Identification of thesein vivoV(D)J mediated inter-chromosomal insertions at ψ RSSs in theHPRTgene supports the accumulating evidence that V(D)J recombinase can mediate mutagenic rearrangements in humans with potential pathologic consequences. Published 2006 Wiley-Liss, Inc.