Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:We have previously shown that activation of TGF- signaling is involved in the myoendothelial gap junctiondependent differentiation of PASMCs to a more contractile-like phenotype in PAEC/PASMC touch coculture (21).
Category: