Putative mechanism of immune escape of 6pUPD-positive clones. (A) A schematic diagram of 6pUPD, which is thought to result from a recombination between 2 homologous chromosomes, invariably involving 6pter distally. (B) Breakpoint mapping of 6pUPD in different patients, showing a prominent breakpoint cluster within the HLA class I region. Breakpoints in critical cases are shown by arrows. (C) 6pUPD-positive (+) HSCs permanently lose the relevant HLA class I molecule required for the presentation of the putative antigen and are thereby thought to escape destruction by CTLs. The 6pUPD-mediated mechanism for immune escape in AA is also supported by the presence of multiple independent 6pUPD clones affecting the same parental HLA allele in some patients.⁶³