Abstract: The present invention relates to a method of site-directed integration into a genetic locus encoding at least one activity of the pyrimidine salvage pathway in a host cell, wherein said activity of the pyrimidine salvage pathway is purine/cytosine permease (FcyB), cytosine deaminase (FcyA), uracil-phosphoribosyl-transferase (Uprt), concentrative nucleoside transporter (CntA) or uridine kinase (UK), comprising: a) providing a host cell comprising a functional copy of the genetic locus encoding at least one activity of the pyrimidine salvage pathway; (b) introducing a gene or sequence of interest into said host cell via transformation of an integrative nucleic acid construct which comprises 3? and/or 5? of the gene or sequence of interest flanks being homologous to said genetic locus or which carries a sequence being homologous to said genetic locus of the pyrimidine salvage pathway and thus allowing for a homologous recombination at said genetic locus, wherein said homologous recombination is capable of causin