Our laboratory also studies more basic aspects of human genetics, with a focus on the understanding of copy number variation. Copy number variants (CNVs) are now appreciated to be a major source of variation in our genomes, and may underlie many of the phenotypic differences in human individuals. We have recently developed a set of genetic tools based on CNVs that allow one to define genetic identity in situ. These are FISH probes that correspond to CNVs that exist as completely deleted sequences in some individuals, and are termed polymorphic deletion probes (PDPs). We are currently applying these PDPs to the in situ study of cellular chimerism in transplantation, including chronic rejection and graft versus host disease.