DNA- and RNA-binding protein involved in various processes, such as translational repression, RNA stabilization, mRNA splicing and transcription regulation (PubMed:30135188, PubMed:31399345). Binds preferentially to the 5'-[CU]CUGCG-3' RNA motif and specifically recognizes mRNA transcripts modified by C5-methylcytosine (m5C) (PubMed:31399345). Promotes mRNA stabilization: acts by binding to m5C-containing mRNAs and recruiting pabpc1a, thereby preventing mRNA decay (PubMed:31399345). Plays a role in the maternal-to-zygotic transition in early embryo by binding to m5C-containing maternal mRNAs and preventing their degradation (PubMed:31399345). Also promotes maternal-to-zygotic transition in oocytes and embryos by promoting translation repression; molecular mechanisms governing translation repression are unknown (PubMed:30135188). 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. 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. Binds to promoters that contain a Y-box (5'-CTGATTGGCCAA-3'). Promotes separation of DNA strands that contain mismatches or are modified by cisplatin. Has endonucleolytic activity and can introduce nicks or breaks into double-stranded DNA, suggesting a role in DNA repair. The secreted form acts as an extracellular mitogen and stimulates cell migration and proliferation (By similarity). Interacts with pabpc1a; leading to pabpc1a recruitment on C5-methylcytosine (m5C)-containing mRNAs. Expressed both maternally and zygotically. In the CSD domain, Trp-45 specifically recognizes C5-methylcytosine (m5C) modification through its indole ring. Embryonic lethality caused by early gastrulation arrest: maternal mutant embryos fail to undergo normal cleavage and the maternal-to-zygotic transition (PubMed:30135188, PubMed:31399345). Early embryos show severe developmental arrest at the shield stage in maternal ybx1 knockout embryos, leading to lethality at 8 hours post-fertilization (hpf) (PubMed:31399345). Belongs to the YBX1 family.