Experimental production of haploid mammalian embryos. (A) Normal fertilization results in embryos containing genomic contributions of both parents. During this process the metaphase II arrest of the oocyte is resolved and the second polar body (PB) is extruded leaving the diploid zygote with a haploid set of chromosomes from each parent. (B) Parthenogenetic activation of oocytes can be achieved by treatment with chemicals including Strontium salts or ethanol without fertilization and results in embryos that contain only one haploid set of maternal chromosomes[62, 66]. (C) Similarly, haploid gynogenetic embryos can be constructed by removing the paternal pronucleus from a fertilized zygote by micromanipulation with a glass capillary in the presence of microtubule inhibiting chemicals. (D) Removal of the maternal pronucleus from the fertilized zygote results in androgenetic embryos containing only a haploid paternal genome[64, 65]. Half of these androgenetic embryos containing the Y chromosome and lacking an X chromosome do not develop. (E) An alternative way for producing haploid androgenetic embryos is to enucleate the oocyte and introduce a sperm nucleus[64, 65]. Between 10 to 20% of haploid embryos containing either the maternal or paternal set of chromosomes develop to the blastocyst stage when they can be used for establishing embryonic stem cell lines.