Mammalian early embryogenesis is supported by maternal factors, such as messenger RNA (mRNA) and proteins, produced and accumulated during oogenesis at least up to the stage when zygotic activation commences. These maternal factors are involved in biologically important events such as epigenetic activation, reprogramming, and mitochondrial growth. Most of these maternal mRNAs are degraded by the 2-cell to 4 ~ 8-cell stages. Maternal proteins, which are produced during oogenesis or by the maternal mRNAs, are degraded by the 4 ~ 8-cell stage. In other words, the maternal factors exist during specific stages of early embryogenesis. In this chapter, we will briefly summarize the property of these maternal factors and mention possible applications of these factors for developing new reproduction engineering-related technologies and producing genetically modified animals. More specifically, we will show the usefulness of maternally accumulated Cas9 protein as a promising tool for CRISPR-/Cas9-based simultaneous genetic modification of multiple loci in mammals.