We characterized the impact G4 formation for TMPRSS2 expression using a breast cancer cell line (MCF-7) and a colorectal cancer cell line (Caco-2), because both cell lines were described to stably express TMPRSS2 even in unchallenged (i.e., without viral infection) conditions (data: Human Protein Atlas, accessed at http://www.proteinatlas.org). Here, TMPRSS2 is linked to breast carcinoma.