Similarly, deficiency of a DNA-dependent protein kinase catalytic subunit (DNA-PKcs; encoded by Prkdc gene) causes a SCID phenotype that is characterized by an absence of functional B and T cells, lymphopenia, hypogammaglobulinemia, but a normal hematopoietic microenvironment (Buckley et al. 1997; Puck et al. 1997). Here, PRKDC is linked to severe combined immunodeficiency.