Lipid compounds and lipid nanoparticle compositions

A compound of formula (I), or a salt or a stereoisomer thereof,

• wherein
• R1 is
• R1a and R1b are each independently —CH3;
• R2 and R3 are each independently H;
• R4 is C10, C11 or C12 linear alkyl;
• R5 and R6 are identical and are selected from C6, C7 and C8 linear alkyl;
• X and Y are each independently O;
• X1 and X2 are independently selected from C═O and O, Y1 and Y2 are independently selected from C═O and O, with the proviso that X1 and Y1 are not both C═O or O, and X2 and Y2 are not both C═O or O;
• m is 0, and n is 1;
• o is 5 or 6;
• p is 7 or 8, and
• q is 2, 3 or 4.