M protein

M protein may refer to:
 * M protein (Streptococcus), a virulence factor of the bacterium Streptococcus pyogenes
 * Viral matrix protein, structural protein linking the viral envelope with the virus capsid
 * Coronavirus membrane protein, structural protein expressed from the M gene in coronaviruses
 * Myeloma protein, also called paraprotein, an abnormal protein in the urine or blood, often seen in multiple myeloma or monoclonal gammopathy of undetermined significance
 * MYOM2 (Myomesin-2), a protein composing the M-line of muscle cell sarcomeres
 * Protein M, immunoglobulin-binding protein found on the surface of the bacterium Mycoplasma genitalium