NPGS is primarily a federally and state-supported effort aimed at maintaining supplies of plant germplasm with diverse genetic traits for use in breeding and scientific research. The diversity in germplasm collections enables breeders to develop improved crops that are more productive and often less vulnerable to pests and diseases. These collections are particularly important because the diversity of germplasm worldwide has been reduced by several factors, such as the widespread use of genetically uniform crops in commercial agriculture and the destruction of natural habitats that have been important sources of germplasm. The Agricultural Marketing Act of 1946 established the main components of NPGS as well as a legal basis for federal and state cooperation in managing plant genetic resources. NPGS' current organizational structure-a geographically dispersed network of germplasm collections administered primarily by USDA'S Agricultural Research Service (ARS)-merged in the early 1970s. NPGS maintains about 440,000 germplasm samples for over 85 crops at 22 sites throughout the country and in Puerto Rico; almost half of these samples are maintained at four regional plant introduction stations. Germplasm samples are held in crop collections, each of which generally includes four types of germplasm (for example, germplasm from cultivated plants and germplasm from wild relatives of cultivated plants). Each type of germplasm contains genetic material that plays an important role in the collections' overall diversity.