In vitro method for producing Neisseria meningitidis capsular polysaccharides which have a defined length, said method comprising the steps as defined under or: (a) incubating at least one capsule polymerase with at least one donor carbohydrate and at least one acceptor carbohydrate; wherein said donor carbohydrate is activated or wherein said donor carbohydrate is activated during step (a); and wherein the ratio of donor carbohydrate to acceptor carbohydrate is a ratio from 10:1 to 400:1, preferably from 20:1 to 400:1; and (b) isolating the resulting capsular polysaccharides, wherein the capsule polymerase is a C-terminally or C-and N-terminally truncated version of the capsule polymerase of Neisseria meningitidis serogroup X consisting of the amino acid sequence of SEQ ID NO: 28 or 29; or of an amino acid sequence having at least 80% identity to SEQ ID NO: 28 or 29, and wherein the capsule polymerase can be regulated via the donor-acceptor ratio; or (a) incubating at least one capsule polymerase with at least one donor carbohydrate and at least one acceptor carbohydrate; wherein said donor carbohydrate is activated or wherein said donor carbohydrate is activated during step (a); and wherein (i) the ratio of donor carbohydrate to acceptor carbohydrate is a ratio from 10:1 to 10000:1, preferably from 200:1 to 1000:1, and (ii) the incubation time ranges from 3 to 45 minutes; and (b) isolating the resulting capsular polysaccharides, wherein the capsule polymerase is a C-terminally or C-and N-terminally truncated version of the capsule polymerase of Neisseria meningitidis serogroup X consisting of the amino acid sequence of SEQ ID NO: 28 or 29; or of an amino acid sequence having at least 80% identity to SEQ ID NO: 28 or 29, and wherein the capsule polymerase can be regulated by the reaction time. (a) incubating at least one capsule polymerase with at least one donor carbohydrate and at least one acceptor carbohydrate; wherein said donor carbohydrate is activated or wherein said donor carbohydrate is activated during step (a); and wherein the ratio of donor carbohydrate to acceptor carbohydrate is a ratio from 10:1 to 400:1, preferably from 20:1 to 400:1; and (b) isolating the resulting capsular polysaccharides, wherein the capsule polymerase is a C-terminally or C-and N-terminally truncated version of the capsule polymerase of Neisseria meningitidis serogroup X consisting of the amino acid sequence of SEQ ID NO: 28 or 29; or of an amino acid sequence having at least 80% identity to SEQ ID NO: 28 or 29, and wherein the capsule polymerase can be regulated via the donor-acceptor ratio; or (a) incubating at least one capsule polymerase with at least one donor carbohydrate and at least one acceptor carbohydrate; wherein said donor carbohydrate is activated or wherein said donor carbohydrate is activated during step (a); and wherein the ratio of donor carbohydrate to acceptor carbohydrate is a ratio from 10:1 to 400:1, preferably from 20:1 to 400:1; and (b) isolating the resulting capsular polysaccharides, (a) incubating at least one capsule polymerase with at least one donor carbohydrate and at least one acceptor carbohydrate; wherein said donor carbohydrate is activated or wherein said donor carbohydrate is activated during step (a); and wherein (i) the ratio of donor carbohydrate to acceptor carbohydrate is a ratio from 10:1 to 10000:1, preferably from 200:1 to 1000:1, and (ii) the incubation time ranges from 3 to 45 minutes; and (b) isolating the resulting capsular polysaccharides, (a) incubating at least one capsule polymerase with at least one donor carbohydrate and at least one acceptor carbohydrate; wherein said donor carbohydrate is activated or wherein said donor carbohydrate is activated during step (a); and wherein (i) the ratio of donor carbohydrate to acceptor carbohydrate is a ratio from 10:1 to 10000:1, preferably from 200:1 to 1000:1, and (ii) the incubation time ranges from 3 to 45 minutes; and (i) the ratio of donor carbohydrate to acceptor carbohydrate is a ratio from 10:1 to 10000:1, preferably from 200:1 to 1000:1, and (ii) the incubation time ranges from 3 to 45 minutes; and (b) isolating the resulting capsular polysaccharides,