Constitutively active ALK2 receptor mutants require type II receptor cooperation. by Bagarova, J, Vonner, A, Armstrong, K, Börgermann, J, Lai, C, Deng, D, Beppu, H, Alfano, I, Filippakopoulos, P, Morrell, N, Bullock, A, Knaus, P, Mishina, Y, Yu, P