Human embryonic stem cells (hESCs) are isolated from the early human embryo and have the capability to proliferate indefinitely in culture and to develop into nearly every cell of the human body. hESC are important for studying developmental biology and for cell replacement therapies for the treatment of degenerative human diseases. An additional use for embryonic stem cells would be for the in vitro study of diseases. hESC lines derived from embryos diagnosed as abnormal by PGD testing would afford such models for study. Because embryos tested by PGD and found to be abnormal would only under rare circumstances be transferred to the uterus of a woman (and in most cases would be discarded), the derivation of new hESC lines from these embryos would provide a viable less ethically-objectionable source of cells with which to study the mechanisms of differentiation and developmental biology.