A process for producing a mutation in an Intended Mutagenesis Region within a bacterial genome, wherein the bacteria are of the class Clostridia, wherein the bacteria comprise a CRISPR/Cas system, and wherein the IMR comprises a CRISPR PAM/Protospacer which is capable of being recognised by the bacteria's CRISPR/Cas systems, the process comprising the steps: (a) transforming a population of said bacteria with a Recombination Vector, wherein the Recombination Vector comprises a Recombination Element, wherein the Recombination Element comprises: (i) a Substitution Element, wherein the Substitution Element comprises the mutation, and (ii) Homology Arms which flank the Substitution Element, wherein the Homology Arms are capable of promoting the replacement of all or part of the IMR in the bacterial genome with an element which comprises the Substitution Element, wherein the Recombination Element does not comprise a CRISPR PAM/Protospacer which is capable of being recognised by a crRNA which recognises the CRISPR/PAM Protospacer in the IMR; (b) culturing the population of bacteria under conditions wherein, in one or more bacteria within the population, all or part of the IMR in the genomes of those bacteria is replaced by an element which comprises the Substitution Element and whereby the CRISPR PAM/Protospacer is removed from the IMR in the genomes of those bacteria or is rendered incapable of being recognised by a crRNA which recognises the CRISPR/PAM Protospacer in the IMR; (c) transforming the population of bacteria with a Killing Vector which is capable of directing production of a crRNA which targets the CRISPR PAM/Protospacer in the IMR of any bacteria in the population from which the CRISPR PAM/Protospacer has not been removed or rendered incapable of being recognised by the crRNA, thereby promoting the CAS endonuclease-induced cleavage of those CRISPR PAM/Protospacers in the genomic DNA which are recognised by the crRNA and the subsequent death of those bacteria; and (d) selecting or isolating one or more bacteria from the population whose genomes comprise the Substitution Element comprising the mutation. (a) transforming a population of said bacteria with a Recombination Vector, wherein the Recombination Vector comprises a Recombination Element, wherein the Recombination Element comprises: (i) a Substitution Element, wherein the Substitution Element comprises the mutation, and (ii) Homology Arms which flank the Substitution Element, wherein the Homology Arms are capable of promoting the replacement of all or part of the IMR in the bacterial genome with an element which comprises the Substitution Element, wherein the Recombination Element does not comprise a CRISPR PAM/Protospacer which is capable of being recognised by a crRNA which recognises the CRISPR/PAM Protospacer in the IMR; (i) a Substitution Element, wherein the Substitution Element comprises the mutation, and (ii) Homology Arms which flank the Substitution Element, wherein the Homology Arms are capable of promoting the replacement of all or part of the IMR in the bacterial genome with an element which comprises the Substitution Element, (b) culturing the population of bacteria under conditions wherein, in one or more bacteria within the population, all or part of the IMR in the genomes of those bacteria is replaced by an element which comprises the Substitution Element and whereby the CRISPR PAM/Protospacer is removed from the IMR in the genomes of those bacteria or is rendered incapable of being recognised by a crRNA which recognises the CRISPR/PAM Protospacer in the IMR; (c) transforming the population of bacteria with a Killing Vector which is capable of directing production of a crRNA which targets the CRISPR PAM/Protospacer in the IMR of any bacteria in the population from which the CRISPR PAM/Protospacer has not been removed or rendered incapable of being recognised by the crRNA, thereby promoting the CAS endonuclease-induced cleavage of those CRISPR PAM/Protospacers in the genomic DNA which are recognised by the crRNA and the subsequent death of those bacteria; and (d) selecting or isolating one or more bacteria from the population whose genomes comprise the Substitution Element comprising the mutation.