There is provided a nucleus-implanted egg that is a nucleus-implanted egg having 2 haploid genome sets derived from mammal ova and shows excellent production efficiency of adults. There is also provided a method of constructing a nucleus-implanted egg having haploid genome sets derived from an ng ovum and an fg ovum, which comprises the steps of (1) introducing a non-grown stage egg (ng ovum) into a nucleus-deleted egg in a germinal vesicle stage (GV stage egg) and then developing them to an MII phase (second meiosis metaphase) by in vitro culture for development to prepare a first nucleus-implanted egg, and (2) extracting MII phase chromosomes from said first nucleus-implanted egg and introducing it into other MII phase egg (fg ovum) to prepare a second nucleus-implanted egg, wherein an ovum in which both (A) a region (region A) that controls expressions of H19 gene and Igf2 gene and (B) a region (region B) that controls expressions of Gtl2 gene and Dlk1 gene are missing is used as the ng ovum or fg ovum, and an adult production rate is significantly improved from that of a conventional method.
