Abstract: The present invention provides a lipid of the formula (I) R3R4N—[Y]q—(CpH2p)—X-Linker-NR1R2 (I) wherein R3 and R4 are independently selected from H and hydrocarbyl groups; q is an integer selected from 1 to 10; Y represents a group (CnH2n)NR5, wherein (i) when q is 1, n is 2, or (ii) when q is greater than 1, each Y may be the same or different and each n is an integer independently selected from 1 to 10, with the proviso that for at least one unit Y, n is 2, and (iii) each R5 is independently selected from H and hydrocarbyl groups; p is an integer selected from 1 to 10; X is an optional group selected from —NR7—, —C(?O)—NR8—, —NR9—C(?O)—, —C(?O)—, —O—, and NR10—C(=0)0-, wherein each R7, R8, R9 and R10 is independently selected from H and hydrocarbyl groups, Linker is an optional group selected from amino acid residues, peptide residues and groups of the formula —(OCH2CH2)1-10, —NR6-(CvH2v)—C(?O)—, wherein R6 is H or a hydrocarbyl group and v is an integer selected from 1 to 11, and —C(=0)-(CH2)0-10—CH2—C(=0)