The importance of the thymus in the immune system was discovered in 1961 by Jacques Miller, by surgically removing the thymus from one day old mice, and observing the subsequent deficiency in a lymphocyte population, subsequently named T cells after the organ of their origin. Until the discovery of its immunological role, the thymus had been dismissed as a "evolutionary accident", without functional importance. The role the thymus played in ensuring mature T cells tolerated the tissues of the body was uncovered in 1962, with the finding that T cells of a transplanted thymus in mice demonstrated tolerance towards tissues of the donor mouse. B cells and T cells were identified as different types of lymphocytes in 1968, and the fact that T cells required maturation in the thymus was understood. The subtypes of T cells (CD8 and CD4) were identified by 1975. The way that these subclasses of T cells matured - positive selection of cells that functionally bound to MHC receptors - was known by the 1990s. The important role of the AIRE gene, and the role of negative selection in preventing autoreactive T cells from maturing, was understood by 1994.