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:After the avian and mammalian lineages diverged, there were further tandem duplications of the π/ζ and αA lineages to produce more complex marsupial and eutherian ('placental') mammalian α-globin clusters, 5'-ζ-ψζ'-αD-ψα3-α2-α1-θ-3' (see [12,15-18]).
Category: