Severe Combined Immunodeficiency is a primary immune deficiency. The defining characteristic is usually a severe defect in both the T- & B-lymphocyte systems. This usually results in the onset of one or more serious infections within the first few months of life. There are at least 13 different genetic defects that can cause SCID. These defects lead to extreme susceptibility to very serious infections. The most common form of SCID is caused by a mutation in the SCIDX1 gene located on the X chromosome.