Interferon-γ signaling mediates transcriptional repression of the perlecan gene. This was first shown in colon cancer cell lines, and subsequently in cell lines of other tissue origins, but in each case intact STAT1 transcription factor was required for the signal to take effect. This led the investigators to believe that the transcription factor STAT1 was interacting with the Pln promoter in the distal region, localized to 660 base pairs upstream of the transcription start site. Interferon- γ treatment of blastocyst-stage murine embryos leads to a loss of perlecan expression on the trophectoderm, and thus an embryonic morphology and phenotype in cell culture, which is suggestive that these interferon-γ treated blastocysts would be defective in implantation. Presumably the loss of perlecan expression stems from downregulation of transcription via STAT1 transcription factor activity as shown previously. These in vitro results are not necessarily representative of normal physiological interferon- γ concentrations, nor are the cytokine normally expressed widely but instead at very specific developmental timepoints. Important to note is that perlecan expression can be decreased by treatment with an exogenous cytokine such as interferon- γ, and if there were a physiologically abnormal increase in expression of the cytokine it could interfere with implantation.