Abstract: Severe Combined Immunodeficiency Disease (SCID) is an autosomal recessive hereditary disease occurring among Arabian horses. The genetic defect responsible for this disease was recently identified as a 5-basepair deletion in the gene encoding DNA-Protein Kinase catalytic subunit (DNA-PKcs). Horses with one copy of the gene appear normal while horses with two copies of the gene manifest the disease. The only way to avoid the economic losses is early detection of SCID carriers. Because the real value of this test is that breeders can use it to ensure that they will never produce a SCID foal and that they can by breeding carriers to clears, preserve the unique and positive characteristics of exceptional carriers. And thereby breed out the undesirable gene over several generations.