The present invention discloses methods for cell nuclear transfer that comprise for example modification of zona pellucida of an oocyte, and/or sectioning of oocytes into several parts. The present invention also discloses methods for producing a genetically modified non-human mammal. Genetically modified non-human mammals obtainable by the disclosed methods are also within the scope of the present invention. Disclosed are also methods for cryopreservation of cells.
