Acquired Uniparental Disomy in a Leukemic Blast in a Patient in Relapse Following Haploidentical Transplant. In the example illustrated, the original mismatched HLA alleles were located on the paternal (P) chromosome 6. During mitosis, a recombinant event occurred in one of the leukemic blasts in which a portion of the long arm of chromosome 6 containing the HLA loci was exchanged between a maternal and a paternal chromosome 6. Subsequently, a leukemic blast derived from this process contained the original maternal (M) chromosome 6 (orange) paired with the recombinant paternal chromosome 6 (green). That particular leukemic blast acquired a survival advantage in the setting of haplounidentical transplantation because it no longer expressed the mismatched paternal HLA proteins, thereby escaping immune surveillance by donor T cells that were haploidentical at the maternal HLA loci. Such recombinant events are not detected by standard karyotyping because there is no observable net loss or gain of genetic material. However, this process can be identified by using single nucleotide polymorphism-array analysis and is called copy number neutral loss of heterozygosity.