It is a form of mutation that leads to either a trinucleotide or dinucleotide expansion, or sometimes contraction, during DNA replication. A slippage event normally occurs when a sequence of repetitive nucleotides (tandem repeats) are found at the site of replication. Tandem repeats are unstable regions of the genome where frequent insertions and deletions of nucleotides can take place, resulting in genome rearrangements. DNA polymerase, the main enzyme to catalyze the polymerization of free deoxyribonucleotides into a newly forming DNA strand, plays a significant role in the occurrence of this mutation. When DNA polymerase encounters a direct repeat, it can undergo a replication slippage.