Nudear transfer (NT) technique provides a powerful experimental tool to study the mechanisms of reprogramming processes and to derive NT-embryonic stem (ntES) cells from living or frozen animals. The Piezo-driven direct microinjcction NT method has proved to be a valid technique to clone mice and otber species. In addition, this method has been broadly used as a versatile tool for many fields of mouse micromanipulation. This chapter describes the "one step method" protocol of nuclear transfer in mouse, which combines injection of a donor cell nucleus and enucleation of MII metaphase in a single manipulation procedure. This protocol describes the isolation and collection of oocytes, treatment of donor cells, visualization of spindle-chromosomal complex, direct injection and enucleation, activation of reconstructed embryos and their in vitro culture and transfer into pseudopregnant mice.