Abstract: A rapid method for transfection of a cell under physiological conditions suitable to the survival and growth of the cell is disclosed. According to the method, a stable nucleoprotein complex is provided. The nucleoprotein complex comprises a single-stranded DNA sequence in stable combination with RecA protein molecules. Cells to be transformed are cultured in a physiologically suitable medium to which the nucleoprotein complex has been added. As the cells grow and undergo mitosis, the nucleoprotein complex is taken up within some of the cells and becomes integrated into the genome. The method accomplishes transfection without resort to infectious vectors or permeabilization or other manipulation of the cell membrane. According to another object of the invention, a diagnostics method is provided. A directly detectable reporter label or an indirectly detectable ligand is bound to the nucleoprotein complex to provide a DNA probe which then is taken up into the cell and integrates into the cell's genome.