Before understanding the gene therapy in SCID patients, we must know more about this immunodeficiency syndrome. SCID stems from a defective lymphoid development. It mainly affects the T-cells, B cells and the natural killer cells (NK cells). Although SCID is an immunodeficiency disorder, it mainly arises due to the gene defects. The circulating lymphocytes in SCID individuals are very less in number. Thus, the entire immune system gets hampered. The failure of the T-cell response and development directly affects the thymus. The thymus is a specialized primary lymphoid organ, important for the immune system. The T-cell maturation mainly occurs in the thymus. They mainly help in building the adaptive immunity and fight with the foreign invaders. The thymus poorly develops in the individuals with the condition such as SCID. The development of the thymus gets affected in this condition. The individuals with SCID experience a low adaptive immunity and hence face the difficulty in fighting the invading microbes. The T-cell count is lower than the expected range. Although the individuals with SCID exhibit normal levels of the myeloid and erythroid cells, the impact of the depleted lymphoid cells is extremely high. Hence, it leads to severe complications.