In early 1995, an M.D-Ph.D student, Jennifer Brown, was breeding mice with an inactivated form of the gene FosB. With the inactivation of the gene, healthy pups from the mutated mice died quickly. Observing this occurrance, Brown found that the mother mice ignored her offspring. From this discovery, Brown proposed that the inactivation of the immediate early gene FosB causes a defect in the nurturing behaviors of female mice. To prove this, Brown bred a series of knockout mice with the inactivated FosB gene. She then observed the nurturing behaviors of the knockout mice and compared them to those of the normal mice.