Since this book is geared to be used by varied groups of readers such as advanced students and instructors in the fields of biology and medicine, scientists and more importantly clinicians, it is considered important to provide brief accounts of the basics of DNA damage, repair, mutagenesis and cancer. The purpose of this book is to present an updated detailed account of some important additional diseases of DNA repair. It has not been possible to cover all the DNA repair deficient diseases in this volume, hence diseases such as Bloom’s syndrome, Werner’s syndrome, Nijmegen breakage syndrome, ataxia telangiectasia‑like disorder, RA D 50 deficiency, RIDDLE syndrome and others will be presented in a forthcoming volume.