 However, the ABCG or White subfamily with its five fully characterized human members consists of half-size ABC proteins which probably dimerize to form active membrane transporters.In our case, it is suggested that ABCG5 and ABCG8 form a heterodimer .Among the half-size molecules, ABCG proteins have a peculiar domain organization characterized by a nucleotide-binding domain at the N-terminus followed by six transmembrane-spanning domains.