The Phage 21 S (P21 Holin) Family (TC# 1.E.1) is a member of the Holin Superfamily II. The Bacteriophage P21 Lysis protein S holin (TC# 1.E.1.1.1) is the prototype for class II holins. Lysis S proteins have two transmembrane segments (TMSs), with both the N- and C-termini on the cytoplasmic side of the inner membrane. TMS1 may be dispensable for function. A representative list of proteins belonging to the P21 holin family can be found in the Transporter Classification Database.