Cells from different organisms can be fused in culture by somatic cell
   hybridization  to  form  a  hybrid  cell  with  two  nuclei  called  a
   heterokaryon;  the  nuclei  can  fuse  to  form  a  synkaryon in which
   chromosomes  from  from  one of the two parental species are gradually
   lost.

   A  panel  of  synkaryons with mostly mouse chromosomes and a few human
   chromosomes  can be used to associate a particular gene product with a
   specific chromosome.

<LI> A  synteny  testing example: how can products A, B, C, D be mapped
to chromosomes?
<li> Product A can be assigned to chromosome 5.
+ Product B can be assigned to chromosome 3.
+ Product C is not on any of the chromosomes 1-7.
+ Product D can be assigned to chromosome 1.