It is the phenomenon where chromosomes are doubled to produce tetraploidy.
Tetraploidization occurs in small but significant quantities even in normal cells, due to a response to minor defects in segregation of chromosomes.
Whereas, the frequency of tetraploidization may get highly increased in tumours due to mutations in regulatory proteins that regulate late M-phase events.
For example, overexpression of the protein kinase aurora A, might cause abortive mitosis which can lead to tetraploidization of cultured mammalian cells.
Aurora A is also seemed to be found expressed overly in many breast cancers.
Mutational defects in the spindle checkpoint system can also be found in many cancer cells and might lead to chromosome-segregation defects that cause tetraploidy.