In this paper we characterize and study a new class of regular Boolean functions called D-reducible. A Dreducible function, depending on all its n input variables, can be studied and synthesized in a space of dimension strictly smaller than n. A D-reducible function can be efficiently decomposed, giving rise to a new logic form, that we have called DRedSOP. This form is often smaller than the corresponding minimum SOP form. Experimental evidence shows that such functions are rather common and D-reducibility can be tested very quickly.