Time course of miR-210 and miR-135b expression under hypoxic conditions. Parental cells (RPMI8226) and HR-MM cells (RPMI8226-HR) were cultured under normoxic conditions (20% O₂) or hypoxic conditions (1% O₂) for the indicated times (0, 24, 48, or 72 hours). Cellular and exosomal miRNA expression levels were quantified by quantitative reverse-transcription polymerase chain reaction: (A) cellular miR-210, (B) exosomal miR-210, (C) cellular miR-135b, and (D) exosomal miR-135b. RNU6B and ath-miR-159 were used as the invariant control for cell and exosome, respectively. Values are means ± SD of 3 independent experiments, with each performed on different days.