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 erythropoietic growth factors affect the progenitors in all these locations [3], and many procedures have been undertaken to reproduce the erythroid maturation in vitro including initial selection of the CD34+ cells [7–11], adherence depletion [1,3,12,13] and phased culturing [6,12,14].
Category: