A method for carrying out recombination at a target locus to add a new sequence, replace an existing sequence and/or deleting an existing sequence at the target locus, which method comprises: - providing two or more nucleic acids which, when taken together as a group, comprise: (a) sequences capable of homologous recombination with sequences flanking the target locus; (b) two or more site-specific recombination sites; (c) a sequence encoding a recombinase which recognizes the site-specific recombination sites; and (d) a sequence encoding a marker, wherein none of the two or more nucleic acids comprises all of the sequences set out in (a), (b), (c) and (d), wherein the two or more nucleic acids are capable of homologous recombination with each other so as to give rise to a single nucleic acid, wherein the ends of the nucleic acids are so designed that the nucleic acids are assembled in a desired orientation and order to yield said single nucleic acid, and wherein at least two of the two or more nucleic acids each comprise a sequence encoding a non-functional portion of the marker; and - introducing the two or more nucleic acids into the host cell so that recombination into said host cell can take place, and - recombining the said two or more nucleic acids with each other and with the sequences flanking the target locus so that a contiguous nucleic acid sequence encoding a functional marker, the sequence encoding the recombinase and at least two site-specific recombination sites are inserted at the target locus, said marker-encoding and/or recombinase-encoding sequence being flanked by at least two site-specific recombination sites and the said site-specific recombination sites being flanked by the sequences capable of homologous recombination with sequences flanking the target locus, wherein the recombination of the nucleic acids with each other and with the sequences flanking the target locus is carried out in vivo in a fungal host cell, wherein: the site-specific recombination sites are lox sites and the recombinase is Cre; or the site-specific recombination sites are FRT sites and the recombinase is Flp; or the recombination sites are Vlox sites and the recombinase is VCre; or the recombination sites are Slox and the recombinase is SCre. - providing two or more nucleic acids which, when taken together as a group, comprise: (a) sequences capable of homologous recombination with sequences flanking the target locus; (b) two or more site-specific recombination sites; (c) a sequence encoding a recombinase which recognizes the site-specific recombination sites; and (d) a sequence encoding a marker, wherein none of the two or more nucleic acids comprises all of the sequences set out in (a), (b), (c) and (d), wherein the two or more nucleic acids are capable of homologous recombination with each other so as to give rise to a single nucleic acid, wherein the ends of the nucleic acids are so designed that the nucleic acids are assembled in a desired orientation and order to yield said single nucleic acid, and wherein at least two of the two or more nucleic acids each comprise a sequence encoding a non-functional portion of the marker; and - introducing the two or more nucleic acids into the host cell so that recombination into said host cell can take place, and - recombining the said two or more nucleic acids with each other and with the sequences flanking the target locus so that a contiguous nucleic acid sequence encoding a functional marker, the sequence encoding the recombinase and at least two site-specific recombination sites are inserted at the target locus, said marker-encoding and/or recombinase-encoding sequence being flanked by at least two site-specific recombination sites and the said site-specific recombination sites being flanked by the sequences capable of homologous recombination with sequences flanking the target locus, wherein the recombination of the nucleic acids with each other and with the sequences flanking the target locus is carried out in vivo in a fungal host cell, wherein: the site-specific recombination sites are lox sites and the recombinase is Cre; or the site-specific recombination sites are FRT sites and the recombinase is Flp; or the recombination sites are Vlox sites and the recombinase is VCre; or the recombination sites are Slox and the recombinase is SCre.