DNA- and RNA-binding protein involved in various processes, such as translational repression, RNA stabilization, mRNA splicing and transcription regulation (By similarity). Binds preferentially to the 5'-[CU]CUGCG-3' RNA motif and specifically recognizes mRNA transcripts modified by C5-methylcytosine (m5C) (By similarity). Promotes mRNA stabilization: acts by binding to m5C-containing mRNAs and preventing mRNA decay (By similarity). Plays a role in the maternal-to-zygotic transition in early embryo by binding to m5C-containing maternal mRNAs and preventing their degradation (By similarity). Also promotes maternal-to-zygotic transition in oocytes and embryos by promoting translation repression; molecular mechanisms governing translation repression are unknown (By similarity). Plays a key role in RNA composition of extracellular exosomes by defining the sorting of small non-coding RNAs, such as tRNAs, Y RNAs, Vault RNAs and miRNAs (By similarity). Probably sorts RNAs in exosomes by recognizing and binding C5-methylcytosine (m5C)-containing RNAs (By similarity). Acts as a key effector of epidermal progenitors by preventing epidermal progenitor senescence: acts by regulating the translation of a senescence-associated subset of cytokine mRNAs, possibly by binding to m5C-containing mRNAs (By similarity). Also involved in pre-mRNA alternative splicing regulation: binds to splice sites in pre-mRNA and regulates splice site selection (By similarity). Also able to bind DNA and regulate transcription (PubMed:2247479). Binds to promoters that contain a Y-box (5'-CTGATTGGCCAA-3') (PubMed:2247479). Promotes separation of DNA strands that contain mismatches or are modified by cisplatin (By similarity). Has endonucleolytic activity and can introduce nicks or breaks into double-stranded DNA, suggesting a role in DNA repair (By similarity). The secreted form acts as an extracellular mitogen and stimulates cell migration and proliferation (By similarity). In the CSD domain, Trp-43 specifically recognizes C5-methylcytosine (m5C) modification through its indole ring. Belongs to the YBX1 family.