Genetically modified mice and nucleic acid constructs for making the genetically modified mice are described. A first mouse having a gene encoding an activator (such as a Cre recombinase) operably linked to a developmentally-regulated promoter (such as a Nanog promoter) is provided. A second mouse having a toxic responder gene (such as a gene encoding diphtheria toxin A) is provided, where the toxic gene is expressed only in the presence of an activator, Embryos from a mating of the first and the second mouse are provided as host embryos suitable for generating mice from donor cells introduced into the host embryos. Ablating the ICM of a mouse embryo physically, chemically, or genetically is described, as well as making F0 generation mice that are substantially or in full derived from donor cells, employing a host mouse embryo with an ablated or nonproliferating ICM.