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 having shown a protective role for CD39 in ALI, we next pursued the functional contributions of CD73 which catalyzes the final conversion of AMP to adenosine and is considered the pacemaker for extracellular adenosine generation (9, 15, 23).
Category: