We prove a uniqueness theorem for irreducible non-critical Gelfand-Tsetlin modules. The uniqueness result leads to a complete classification of the irreducible Gelfand-Tsetlin modules with 1-singularity. An explicit construction of such modules was given in [8]. In particular, we show that the modules constructed in [8] exhaust all irreducible Gelfand-Tsetlin modules with 1-singularity. To prove the result we introduce a new category of modules (called Drinfeld category) related to the Drinfeld generators of the Yangian Y (gl n ) and define a functor from the category of non-critical Gelfand-Tsetlin modules to the Drinfeld category.1991 Mathematics Subject Classification. Primary 17B67.Henceforth, we define V (L) to be the complex vector space with basis the set L Z , i.e.An open problem which dates back to the work of Gelfand and Graev, [11], is to define a gl n -module structure on a subspace of V (L) annihilated by some maximal ideal of the Gelfand-Tsetlin subalgebra. Solutions to this problem are known in various cases, the most fundamental of which relies on the construction of Gelfand-Tsetlin bases of finite dimensional representations of g.Another important solution concerns the generic Gelfand-Tsetlin modules. More precisely, a pair of entries (l mi , l mj ) of L such that l mi − l mj ∈ Z is called a singular pair. If T (L) contains a singular pair in the m-th row for some 2 ≤ m ≤ n − 1 then we call T (L) a singular tableau (and L a singular element in T n (C)). A tableau T (L) is 1-singular if it contains exactly one singular pair. If T (L) is not singular it is called generic. For a generic tableau T (L), one can imitate the construction of Gelfand-Tsetlin bases of finite-dimensional representations and construct a gl nmodule structure on V (L), see for example [5].The study of singular modules V (L) was initiated in [8], where a module structure on V (L) was introduced for any 1-singular tableau L. The latter construction was generalized in [9] for singular tableaux L with multiple singular pairs such that the difference of the entries of any two distinct singular pairs is noniteger. By taking irreducible quotients of V (L) new irreducible Gelfand-Tsetlin modules for gl n were constructed. In particular, understanding the 1-singular case helped us to complete classification of irreducible Gelfand-Tsetlin gl 3 -modules in [10]. Explicit basis for a class of irreducible 1-singular Gelfand-Tsetlin gl n -modules was constructed in [13].We note that to each L ∈ T n (C) we associate the maximal ideal m L of the Gelfand-Tsetlin subalgebra Γ generated by c ij −γ ij (L), 1 ≤ j ≤ i ≤ n, where c ij are the generators of Γ, and γ ij are symmetric polynomials defined in (3). Furthermore, for every maximal ideal m of Γ there exists an irreducible Gelfand-Tsetlin module V such that V m = 0 (see (2) for the definition of V m ) and the number of such non-isomorphic irreducible modules V is finite, [24]. In fact, for a generic L there exists a unique up to isomorphism irreducible module V such that V mL = 0. On the othe...