X-linked immunodeficiencies: clues to genes involved in T- and B-cell differentiation

There are five major human X-linked immunodeficiencies, each with a characteristic impairment of T-and/or B-cell differentiation. The molecular bases of these diseases remain unknown but, as Geneviève de Saint Basile and Alain Fischer report, major steps towards that goal have been taken: the location of the defective genes has been precisely defined and the cell lineages and stages of differentiation in which the genes are expressed have been partly identified.