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:Human leukemia cell lines, HL60 [38] and RPMI8226 [39], were purchased from DS Pharma Biomedical (Osaka, Japan) and cultured in RPMI 1640 medium (Invitrogen, California) containing 10% fetal bovine serum and a 1% penicillin-streptomycin mixture (Sigma) at 37uC under 5% CO2 in a humidified incubator and passaged every three days.
Category: