Collagen I induces matrix metalloproteinase (MMP)-2 expression through discoidin domain receptor (DDR)2, but not DDR1 in normal human lung fibroblasts (NHLFs). NHLFs were reverse transfected with negative control small interfering (si)RNA, and DDR2-specific and DDR1-specific siRNA using lipofectamine RNAiMAX. At 48 h after transfection, NHLFs were serum-starved for 24 h and incubated with collagen I (25 μg/mL) or vehicle (acetic acid, 0.1 M) for 16 h. Total RNA was isolated, reverse transcribed and real-time quantitative PCR was performed using the TaqMan system with specific primers and TaqMan probes for human, MMP-2 (A), and glyceraldehyde 3-phosphate dehydrogenase (GAPDH). The expression changes (fold increase) were calculated relative to unstimulated control cells after normalizing with GAPDH. Results are representative of mean fold increase ± SD of three independent experiments performed in triplicate (n = 9, *P < 0.05). NHLFs were transfected with DDR2-specific or DDR1-specific siRNAs or negative control siRNA prior to starvation and 16 h of collagen I stimulation. MMP-2 (B) protein was measured in the culture supernatant by ELISA. Results are representative of mean fold increase ± SD of three independent experiments performed in triplicate (n = 9, *P < 0.05).