There is described a method of extracting DNA and/or RNA from a cell or capsid, the method comprising contacting the cell or capsid with a composition comprising a quaternary ammonium compound including a silicon-containing functional group. The quaternary ammonium compound may be of general formula (I) or a derivative salt thereof wherein L is a linking group; each of R1, R2, R3, R4, R5 and R6 is independently selected from H or an optionally substituted alkyl, alkenyl, aryl or alkoxy group; and n is 0 or 1. A PCR promoting agent may be provided. The method may be one of 1 detecting and/or diagnosing a disease or medical condition.