https://www.selleckchem.com/products/mtx-211.html In this protocol, we describe the establishment of a CRISPR/Cas9 system in Trichoderma reesei by generating a specific, codon-optimized Cas9-expressing strain and by in vitro transcription of a gRNA. This system induces mutagenesis or introduces a gene in a targeted way based on PEG-mediated protoplast transformation. Up to three targets, multiplexed genome editing can be obtained in one transformation.This chapter describes how mating assays in Trichoderma reesei can successfully be performed and which specific prerequisites of industrial strains originating from strain QM6a have to be met for successful mating experiments.During the electroporation of T. reesei, linearized exogenous DNA is absorbed into swollen conidia by an electrical impulse. The advantage of this method is that it is less time-consuming, less expensive, and easier to perform than the classical protoplast transformation while at the same time having a comparable efficiency.In this chapter, we describe a routinely used strategy for targeted gene insertions in Trichoderma reesei using auxotrophic markers. Generally, targeted gene integrations are advantageous over random, ectopic integration, because the copy number and locus of integration are controlled, abolishing the risk of pleiotropic effects. The use of auxotrophic markers allows a direct, cheap, and easy method for selection. The first step is the construction of recipient strains in a NHEJ-deficient strain. We routinely use deletion strains of pyr4, encoding for the orotidine 5'-phosphate decarboxylase (EC 4.1.1.23) and/or asl1, encoding for the argininosuccinate lyase (EC 4.3.2.1). In the second step, the gene of interest is inserted together with the marker gene. Here we describe the necessary strategy for the construction of the recipient strains and insertion constructs, a PEG-mediated transformation protocol, and a protocol for genetic confirmation of the gene insertion.Transformation