Publisher Summary This chapter highlights the genetic studies concerned with human breast cancer and discusses the background of breast cancer histology and clinical aspects of breast cancer. It also discusses the method by which the genetic changes and the resulting alterations in the encoded protein products can be detected in clinical samples. The types of genetic alteration found in human tumors include oncogene amplification, inactivation of tumor suppressor genes, point mutations, and translocations. Invasive breast carcinoma is usually detected when a lump is felt by the patient or her physician. The gene alterations found in breast carcinomas are mainly amplification of a number region of DNA and loss of heterozygosity for a number of regions of DNA. Genetic changes in human breast cancer can be determined by the amplification of oncogenes, point mutations, cytogenetic studies, and the inactivation of tumor suppressor genes. Breast carcinomas are heterogeneous and can be divided in distinct subgroups: tumors with neu gene amplification and those with amplification of the chromosome 11q13 region. The gene alterations in breast cancer make it possible to devise new types of treatment, directed against the altered gene products. The clinicopathological aspects of breast cancer are also discussed in the chapter.