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CRISPR-Cas are prokaryotic adaptive immune systems. Cas nucleases generally use CRISPR-derived RNA guides to specifically bind and cleave DNA or RNA targets. Here, we describe the experimental characterization of a bacterial CRISPR effector protein Cas12m representing subtype V-M. Despite being less than half the size of Cas12a, Cas12m catalyzes auto-processing of a crRNA guide, recognizes a 5'-TTN' protospacer-adjacent motif (PAM), and stably binds a guide-complementary double-stranded DNA (dsDNA). Cas12m has a RuvC domain with a non-canonical catalytic site and accordingly is incapable of guide-dependent cleavage of target nucleic acids. Despite lacking target cleavage activity, the high binding affinity of Cas12m to dsDNA targets allows for interference as demonstrated by its ability to protect bacteria against invading plasmids through silencing invader transcription and/or replication. Based on these molecular features, we repurposed Cas12m by fusing it to a cytidine deaminase that resulted in base editing within a distinct window., Competing Interests: Declaration of interests Two patent applications have been filed related to this work, with J.v.d.O., P.M., W.Y.W., and S.C.A.C. as inventors. J.v.d.O. is founder and scientific adviser of NTrans Technologies and adviser of Scope Biosciences and Hudson River Biotechnology. W.X.Y. and D.A.S. are employees and shareholders of Arbor Biotechnologies, Inc. C.L.B. is a co-founder and member of the scientific advisory board for Locus Biosciences and is a member of the scientific advisory board for Benson Hill. C.L.B. has submitted patent applications on CRISPR technologies unrelated to this work., (Copyright © 2022 Elsevier Inc. All rights reserved.)