Mice, humans, and most mammals—including dogs, cats, rabbits, monkeys, and apes—have roughly the same number of nucleotides in their genomes: about 3 billion base pairs. This comparable DNA content implies that all mammals contain more or less the same number of genes. Inherited human diseases can be caused by a single alteration in one DNA molecule, leading to inheritance of sickle cell disease, cystic fibrosis, breast cancer, and many other diseases.
Because mouse and humans share about 85 percent of the same genes, we can idendify the gene in a mouse model, modify it (or create mutations), then transfer that mutation from generation to generation of study mice. By targeting these modified genes in mouse models, scientists can study human diseases safely and learn how to treat or prevent certain illnesses.