text: Gene knockouts (also known as gene deletion or gene inactivation) are a widely used genetic engineering technique that involves the targeted removal or inactivation of a specific gene within an organism's genome. This can be done through a variety of methods, including homologous recombination, CRISPR-Cas9, and TALENs.
  One of the main advantages of gene knockouts is that they allow researchers to study the function of a specific gene in vivo, and to understand the role of the gene in normal development and physiology as well as in the pathology of diseases. By studying the phenotype of the organism with the knocked out gene, researchers can gain insights into the biological processes that the gene is involved in.
  There are two main types of gene knockouts: complete and conditional. A complete gene knockout permanently inactivates the gene, while a conditional gene knockout allows for the gene to be turned off and on at specific times or in specific tissues. Conditional knockouts are particularly useful for studying developmental processes and for understanding the role of a gene in specific cell types or tissues.
  Gene knockouts have been widely used in many different organisms, including bacteria, yeast, fruit flies, zebrafish, and mice. In mice, gene knockouts are commonly used to study the function of specific genes in development, physiology, and cancer research.
  The use of gene knockouts in mouse models has been particularly valuable in the study of human diseases. For example, gene knockouts in mice have been used to study the role of specific genes in cancer, neurological disorders, immune disorders, and metabolic disorders.
  However, gene knockouts also have some limitations. For example, the loss of a single gene may not fully mimic the effects of a genetic disorder, and the knockouts may have unintended effects on other genes or pathways. Additionally, gene knockouts are not always a good model for human disease as the mouse genome is not identical to the human genome, and mouse physiology is different from human physiology.
  The KO technique is essentially the opposite of a gene knock-in. Knocking out two genes simultaneously in an organism is known as a double knockout (DKO). Similarly the terms triple knockout (TKO) and quadruple knockouts (QKO) are used to describe three or four knocked out genes, respectively. However, one needs to distinguish between heterozygous and homozygous KOs. In the former, only one of two gene copies (alleles) is knocked out, in the latter both are knocked out.