In the context of endometriosis-related fibrosis, primary human peritoneal mesothelial cells in which Sp/KLF transcription factor Krüppel-like factor 11 (KLF11) was knocked down, treatment with progesterone resulted in an increase in COL1A1 mRNA levels, whereas conversely a decrease of the COL1A1 mRNA levels following treatment with oestrogen was noted. The gene discussed is COL1A1; the disease is endometriosis.