Proteins are removed from the DNA solution by treatment of tissue homogenate with organic solvents such as phenol or the mixture of chloroform-isoamyl alcohol. Upon centrifugation, the denatured proteins form a layer at the interface between aqueous and organic phases, lipids and other contaminants remain in the organic phase while nucleic acids (i.e., DNA and RNA) are recovered in the aqueous phase from which they are precipitate. The contaminating RNA is removed by selective salt precipitation or treatment with DNase-free RNase.