text: CRISPR-associated transposons or CASTs are mobile genetic elements (MGEs) that have evolved to make use of minimal CRISPR systems for RNA-guided transposition of their DNA. Unlike traditional CRISPR systems that contain interference mechanisms to degrade targeted DNA, CASTs lack proteins and/or protein domains responsible for DNA cleavage. Specialized transposon machinery, similar to that of the well characterized Tn7 transposon, complexes with the CRISPR RNA (crRNA) and associated Cas proteins for transposition. CAST systems have been characterized in a wide range of bacteria and make use of variable CRISPR configurations including Type I-F, Type I-B, Type I-C, Type I-D, Type I-E, Type IV, and Type V-K. MGEs remain an important part of genetic exchange by horizontal gene transfer and CASTs have been implicated in the exchange of antibiotic resistance and antiviral defense mechanisms, as well as genes involved in central carbon metabolism. These systems show promise for genetic engineering due to their programmability, PAM flexibility, and ability to insert directly into the host genome without double strand breaks requiring activation of host repair mechanisms.  They also lack Cas1 and Cas2 proteins and so rely on other more complete CRISPR systems for spacer acquisition in trans.