Laboratory mice are special breeds of house mice and comprise a majority of the animals used in research, teaching, and testing. Large mammals are not often practical for scientific study. Mice, however, are small, easy to handle, inexpensive to rear, breed quickly, and are genetically similar to humans. Beginning in the Twentieth Century, scientists bred different strains of mice with specific genetic deficiencies in order to produce models for human diseases. Today, mice are used to study everything from drug addiction to cancer and human immunodeficiency virus (HIV).