What is described is a compound of formula I wherein R1 consists of a linear or branched alkyl consisting of 1-18 carbons, an alkenyl or alkynyl consisting of 2 to 12 carbons, or a cholesteryl; R2 consists of a linear or branched alkyl or an alkenyl consisting of 1 to 18 carbons; L1 consists of a linear alkyl consisting of 5 to 9 carbons or, when R1 consists of a cholesteryl then L1 consists of a linear alkylene or alkenyl consisting of 3 to 4 carbons; X1 consists of —O—(CO)— or —(CO)—O—; X2 consists of S or O; L2 consists of a bond or a linear alkylene of 1 to 6 carbons; R3 consists of a linear or branched alkylene with 1 to 6 carbons; and R4 and R5 are the same or different, each consisting of a linear or branched alkyl of 1 to 6 carbons; or a pharmaceutically acceptable salt thereof.