MicroRNAs mediate down-regulation of target genes via binding to the 3'-untranslated region of mRNA targets followed by degradation of the mRNA or translational inhibition. miR-21 is expressed in many cell types and has several functions, e.g. regulating process connected to cell growth, differentiation, tumor migration and invasion. miR-21 is upregulated in many autoimmune diseases like systemic lupus erythematosus (SLE) and is known to be a key regulator of the transition from a pro-inflammatory to an anti-inflammatory state in inflammatory diseases. Based on our preliminary experiments, miR-21 was found to be significantly upregulated in NC16A-IgG treated HaCaT cells and overexpressed in perilesional skin of bullous pemphigoid (BP) patients. Our hypothesis was, that miR-21 knockout (KO) mice might be protected from antibody-induced BP, but the KO mice (n=5) showed a significantly higher disease activity than control mice (wild type, n=3). We hope to reveal the role of miR-21 in the autoimmune and inflammatory process of BP.