The goal of this paper is two-fold: (a) it argues for the existence of a DP projection in Serbian based on observed selectional dependencies between Ds and relative clauses (RC), and (b) it proposes that Serbian RCs are optional arguments of D (Smith 1964, Larson 1991). Specifically, I offer new arguments in favor of the Universal DP-Hypothesis, examining a key syntactic point that has received little attention in the literature, viz., that lack of a DP projection in article-less languages will require an NP-adjunct analysis of relative clauses. I discuss apparent selectional dependencies between D-elements and relative clauses that strongly undermine this view. These findings thus support the presence of DP in article-less languages, and the broader claim of universality for the functional category set.