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:the common cascade; ligand binds to and activates the respective receptors (activin receptor-like kinase (ALK) 5 as a type I receptor and TbRII as a type II receptor for TGF-b, and ALK4 as a type I receptor and ActRIIA/IIB as a type II receptor for activin), leading to phosphorylation and activation of receptor-regulated Smad, Smad2/3 and subsequent transcriptional regulation in concert with common Smad, Smad4 [4–7].
Category: