Recombination activating gene and its defects.

Mutations in recombination activating genes cause a spectrum of severe immunodeficiencies ranging from T-B severe combined immunodeficiency to Omenn syndrome (a particular type of severe combined immunodeficiency presenting a T+ B- profile). Although environmental factors and genetic background could also contribute to the genesis of this pathological condition, a residual recombination activating gene activity allowing for a few recombinational events to occur, is the first determinant of this variability in the clinical picture.