The present invention relates to methods and compositions for preventing the occurance or progression of a cancer or pre-cancerous condition associated with expression, or over-expression of human cytidine deaminases of the APOBEC3 family. The invention also relates to drug screening assays designed to identify compounds that regulate the activity, or level of expression, of hA3A, hA3C and hA3H. The invention further relates to transgenic mice, as well as cells derived from said mice, that have been genetically engineered to express, or over-express hA3A, hA3C and/or hA3H. Such mice may be utilized to screen for, or identify, compounds that modulate the activity, or expression, of the human cytidine deaminases. The present invention also provides topical compositons such as cosmetic lotion, creme, or sunscreen for use on the skin, which comprise one or more inhibitors of human cytidine deaminase activity. The present invention relates to a double stranded DNA obtained following opening up of its duplex structure, said DNA being edited with cellular protein normally or abnormally expressed in the nucleus of an eukaryotic cell. The mono stranded DNA derived from the said double stranded DNA is a part of the present invention.