The present invention relates to a method for diagnosing lupus nephritis in an individual suffering from systemic lupus erythematosus (SLE), said method conducted in vitro comprising identifying a protein that comprises an YB-1-like cold-shock domain which bears one or more guanidinylated lysinyl moieties (YB-1-G) in a sample S obtained from the individual. Furthermore, the present invention also refers to treating or preventing lupus nephritis in an individual, wherein lupus nephritis has been diagnosed in the individual by means of a method of the present invention.