A gene is a DNA sequence that can be transcribed into an RNA molecule and transferred to offspring organisms. Changes in DNA sequences that determine the structure and function of a gene are called mutations. Gene Mutations: Causes and Effects opens by exploring the physical, chemical, and biological agents that cause mutations interact with DNA, leading to genetic instability. Recent advances in next-generation sequencing have led to the discovery of new causative genes or those mutations. The authors describe the phenotypes and gene mutations, discussing genotype-phenotype correlations compared with previous reports. Lastly, one study analyses all conflicting data concerning the amplification of the ESR1 gene, particularly its ambiguous prevalence in both untreated tumors and tumors either responsive or unresponsive to antiestrogen therapy.