A composition comprising a messenger RNA (mRNA) encoding a protein that is deficient in a subject, wherein the protein encoded by the mRNA: (i) normally functions in the neurons in the brain and/or spinal cord; and/or (ii) normally functions in the motor neurons in the spinal cord, wherein the mRNA is encapsulated within a liposome for use in a method of treating a disease, disorder or condition associated with deficiency of the protein in the central nervous system, the use comprising administering by intrathecal delivery to the subject in need of treatment the composition such that the administering of the composition results in the intracellular delivery of the mRNA in neurons in the brain and/or spinal cord; and wherein the intracellular delivery of the mRNA results in: (i) intracellular expression of the protein encoded by the mRNA within the cytosol of the neurons; and/or (ii) expression of the protein encoded by the mRNA and secretion extracellularly from the neurons after expression; wherein the liposome comprises a cationic lipid, a non-cationic lipid, a cholesterol-based lipid and a PEG-modified lipid in a ratio of 30-60: 25-35: 20-30: 1-15, respectively, wherein the cationic lipid is: (i) a compound of formula I-g: or a pharmaceutically acceptable salt thereof, wherein each R L independently is (a) C 8-12 alkyl, (b) n -C 8-12 alkyl, (c) C 9-11 alkyl, (d) n -C 9-11 alkyl, (e) C 10 alkyl, or (f) n -C 10 alkyl, or (ii) selected from C12-200, DLin-KC2-DMA, cKK-E12, Re-I, DODMA, ICE or (15Z, 18Z)-N,N-dimethyl-6-((9Z, 12Z)-octadeca-9, 12-dien- 1 -yl)tetracosa-4,15,18-trien-1-amine. (i) normally functions in the neurons in the brain and/or spinal cord; and/or (ii) normally functions in the motor neurons in the spinal cord, wherein the mRNA is encapsulated within a liposome for use in a method of treating a disease, disorder or condition associated with deficiency of the protein in the central nervous system, the use comprising administering by intrathecal delivery to the subject in need of treatment the composition such that the administering of the composition results in the intracellular delivery of the mRNA in neurons in the brain and/or spinal cord; and wherein the intracellular delivery of the mRNA results in: (i) intracellular expression of the protein encoded by the mRNA within the cytosol of the neurons; and/or (ii) expression of the protein encoded by the mRNA and secretion extracellularly from the neurons after expression; wherein the liposome comprises a cationic lipid, a non-cationic lipid, a cholesterol-based lipid and a PEG-modified lipid in a ratio of 30-60: 25-35: 20-30: 1-15, respectively, wherein the cationic lipid is: (i) a compound of formula I-g: or a pharmaceutically acceptable salt thereof, wherein each R L independently is (a) C 8-12 alkyl, (b) n -C 8-12 alkyl, (c) C 9-11 alkyl, (d) n -C 9-11 alkyl, (e) C 10 alkyl, or (f) n -C 10 alkyl, or (ii) selected from C12-200, DLin-KC2-DMA, cKK-E12, Re-I, DODMA, ICE or (15Z, 18Z)-N,N-dimethyl-6-((9Z, 12Z)-octadeca-9, 12-dien- 1 -yl)tetracosa-4,15,18-trien-1-amine. wherein the mRNA is encapsulated within a liposome for use in a method of treating a disease, disorder or condition associated with deficiency of the protein in the central nervous system, the use comprising administering by intrathecal delivery to the subject in need of treatment the composition such that the administering of the composition results in the intracellular delivery of the mRNA in neurons in the brain and/or spinal cord; and wherein the intracellular delivery of the mRNA results in: (i) intracellular expression of the protein encoded by the mRNA within the cytosol of the neurons; and/or (ii) expression of the protein encoded by the mRNA and secretion extracellularly from the neurons after expression; (i) intracellular expression of the protein encoded by the mRNA within the cytosol of the neurons; and/or (ii) expression of the protein encoded by the mRNA and secretion extracellularly from the neurons after expression; wherein the liposome comprises a cationic lipid, a non-cationic lipid, a cholesterol-based lipid and a PEG-modified lipid in a ratio of 30-60: 25-35: 20-30: 1-15, respectively, wherein the cationic lipid is: (i) a compound of formula I-g: or a pharmaceutically acceptable salt thereof, wherein each R L independently is (a) C 8-12 alkyl, (b) n -C 8-12 alkyl, (c) C 9-11 alkyl, (d) n -C 9-11 alkyl, (e) C 10 alkyl, or (f) n -C 10 alkyl, or (ii) selected from C12-200, DLin-KC2-DMA, cKK-E12, Re-I, DODMA, ICE or (15Z, 18Z)-N,N-dimethyl-6-((9Z, 12Z)-octadeca-9, 12-dien- 1 -yl)tetracosa-4,15,18-trien-1-amine. (i) a compound of formula I-g: or a pharmaceutically acceptable salt thereof, wherein each R L independently is (a) C 8-12 alkyl, (b) n -C 8-12 alkyl, (c) C 9-11 alkyl, (d) n -C 9-11 alkyl, (e) C 10 alkyl, or (f) n -C 10 alkyl, or (ii) selected from C12-200, DLin-KC2-DMA, cKK-E12, Re-I, DODMA, ICE or (15Z, 18Z)-N,N-dimethyl-6-((9Z, 12Z)-octadeca-9, 12-dien- 1 -yl)tetracosa-4,15,18-trien-1-amine.