Abstract: A method of promoting annealing of two nucleic acid molecules that contain complementary nucleotide sequences is disclosed. The method comprises the step of contacting the nucleic acid molecules in the presence of hnRNP U protein. A kit for hybridizing two nucleic acid molecules is disclosed. Isolated DNA molecules encoding the hnRNP U protein, expression vectors comprising the DNA molecule, host cells transformed with the expression vector and a method of producing essentially pure hnRNP U protein are disclosed. Essentially pure hnRNP U protein is disclosed. Methods of facilitating intermolecular interactions between two molecules in which at least one molecule is a nucleic acid by contacting the molecules in the presence of a protein, such as hnRNP U protein, which comprises a highly conserved "RGG box" amino acid sequence pattern are disclosed.