This invention concerns "knockout" animals, including mice, which have a deficit of functional steroid hormone receptors, DNA constructs containing the mutations, and methods for producing the animals. The mutation is introduced into the animal or its ancestors at an embryonic stage. These knockout animals provide a model system for studying the biological role of hormones, including steroid hormones and sex steroids, in growth, development, morphological differentiation, and sexual and reproductive behavior and cycles, etc.