The function of FASTKD5 (FAST kinase domains 5, Ensembl gene identifier ENSG00000215251) is as follows. Required for the processing of non-canonical mitochondrial pre-mRNAs within the primary mitochondrial polycistronic transcript, i.e. pre-mRNA sequences that are not flanked by tRNA (tRNA punctuation) (PubMed:25683715, PubMed:40499538, PubMed:40637235). Has endoribonuclease activity and processes MT-CO1, MT-CO3 and MT-CYB pre- mRNAs, generating products with 3'/2'-phosphate or a 2',3'-cyclic phosphodiester at the 3'-end (PubMed:40637235). {ECO:0000269|PubMed:25683715, ECO:0000269|PubMed:40499538, ECO:0000269|PubMed:40637235}.