What is described is a compound of formula I consisting of a compound in which R1 is a branched chain alkyl consisting of 10 to 31 carbons; R2 is a linear alkyl, alkenyl, or alkynyl consisting of 2 to 20 carbons; L1 and L2 are the same or different, each a linear alkylene of 1 to 20 carbons or a linear alkenylene of 2 to 20 carbons; X1 is S or O; R3 is a linear or branched alkylene consisting of 1 to 6 carbons; and R4 and R5 are the same or different, each a hydrogen or a linear or branched alkyl consisting of 1 to 6 carbons; or a pharmaceutically acceptable salt thereof.