Precise modifications of the Saccharomyces cerevisiae genome use marker cassettes, most often in the form of “knockout” (KO) marker cassettes, to delete genes. Many different KO marker cassettes exist, some of which require strains with specific genotypes, such as auxotrophic mutations, and others that have no strain genotype requirements, such as selections for drug resistance and one of two selections for nitrogen source utilization. This introduction focuses on the most frequently used family of KO cassettes—the MX cassettes. In particular, we focus on and describe the different types of MX cassettes and selections; specifically, selections for prototrophy; selections for utilization of cytosine or acetamide as sole nitrogen sources; and selections for resistance to six different drugs. The use of cassettes to place genes under regulated control is briefly discussed. Also discussed are strain genotype requirements (where applicable); media requirements; how to “recycle” or “pop out” cassettes; and counterselections against specific KO cassettes.