Classification of CN-LOH. CN-LOH can be classified by either its origin or its location. CN-LOH can have an acquired, clonal derivation or a constitutional, nonclonal derivation. Nonclonal CN-LOH can be the result of an early embryonic mitotic event, leading to mosaicism, or may be truly constitutional. This constitutional CN-LOH can arise from autozygosity or meiotic events, including trisomic rescue, gamete complementation, duplication of a monosomic chromosome in an aneuploid zygote, or nonhomologous recombination. In addition, CN-LOH can be either segmental or numerical. Segmental CN-LOH arising from one crossing over will be telomeric (a, bottom left), whereas 2 crossing-over events will lead to interstitial CN-LOH (b). CN-LOH can also involve an entire chromosome (numeric; c, d, and e).