A mongrel, mixed-breed dog or mutt, is a dog that is not the result of intentional breeding. Estimates place their numbers at 150 million animals. Although the term "mixed-breed dog" is preferred by some, many mongrels have no purebred ancestors. Furthermore, crossbreed dogs, while literally a mix of breeds, differ from mongrels in being intentionally bred. Although mongrels have at times been considered somehow lesser than intentionally bred dogs, they tend to be less susceptible to genetic health problems associated with dog breeding, and have enthusiasts and defenders who prefer them to intentionally bred dogs.