The study focuses on the ability to substitute MHC-defined porcine fetal thymus tissue for human fetal thymus tissue for the construction of human immune system (HIS) mice. The study explores the ability to substitute MHC-defined porcine fetal thymus tissue for human fetal thymus tissue for the construction of humanized immune system (HIS) mice. T cell reconstitution and function in HIS mice constructed with fetal pig thymic tissue will be compared to mice HIS mice constructed from cord blood CD34+ cells, followed by an attempt to improve reconstitution and function by porcine thymus by using pigs that are transgenic for common HLA alleles.