sentences:  Phosphorylated SMAD-2 and -3 then form heteromeric complexes with common-mediator SMAD 4 (co-SMAD-4).Once assembled, the complexes translocates into the nucleus via the nuclear pore with the assistance of importins alpha/beta.Once in the nucleus, these complexes accumulate and eventually act as a transcription factors, regulating the expression of TGF-β target genes.