Congenital thymic aplasia

Aplasia ( (listen); from Greek a, "not", "no" + plasis, "formation") is a birth defect where an organ or tissue is wholly or largely absent. It is caused by a defect in a developmental process.Aplastic anemia is the failure of the body to produce blood cells. [Source: Wikipedia ]

OrphaNet reference
Thymic aplasia 
May Be Caused by
Chromosome 22q11.2 deletion