In Somatic hybrids, human cell can be fused with mouse (some other organism) cells. At first they form heterocaryons (cells with two or more nuclei) but eventually they form hybrid cells (each with one fused nucleus). The treatment of cells with polyethylene glycol or inactivated Sendai virus  is what causes the fusion to occur.  In human-rodent hybrid cells for some reason their is a progressive loss of the human chromosomes.  This feature of hybrid cells allows researchers to isolate a single human chromosome, or even a part of a chromosome.  Often chromosomes that have translocations or deletions that are related to diseases are used in the creation of somatic cell hybrids.  Studying panels of somatic cells hybrids, each with a different translocation or a deletion has facilitated the ability of researchers to narrow down the location of genes that cause disease(9).