The biggest problem by then was having no known method to reproduce hybrid rice in mass quantities, and that was the problem that Yuan set out to solve. In 1964, Yuan created his theory of using a hypothetical naturally-mutated male-sterile strain of rice that he predicted most probably existed for the creation of a new reproductive hybrid rice species, and in two years' time he managed to successfully find a few individuals of such a mutated male-sterile rice that he could use for his research. Subsequent experiments proved his original theory feasible, making that theory his most important contribution to hybrid rice.