The invention provides a method for altering the sequence of a target nucleic acid molecule to incorporate a replacement nucleic acid sequence. The method comprising the steps of: a) introducing a nucleic acid fragment into the target nucleic acid molecule by homologous recombination, wherein the nucleic acid fragment comprises: i) a first region of homology to the target nucleic acid molecule; ii) a 5' splice site; iii) a selectable marker; iv) a 3' splice site; v) a second region of homology to the target nucleic acid molecule; wherein components i) to v) are ordered from 5' to 3'; and the replacement nucleic acid sequence is positioned within or between the first and second regions of homology but external of the splice sites; b) selecting for the introduction of the nucleic acid fragment using the selectable marker; c) incubating the product of step b) under conditions suitable for a splicing reaction to occur such that the selectable marker is excised and from the target nucleic acid, and thereby generating the desired altered target nucleic acid sequence.
