This document summarizes molecular mechanisms of mutation and DNA damage repair in cells. It discusses how mutagens can induce point mutations by substituting or damaging DNA bases, causing errors during DNA replication. It also describes genetic diseases linked to expansions of trinucleotide repeats, such as Fragile X syndrome and Huntington's disease. The document outlines several DNA repair pathways in cells and how defects in these pathways can lead to increased mutation rates and cancer-proneness.