DNA- and RNA-binding protein involved in various processes, such as translational repression, RNA stabilization, mRNA splicing, DNA repair and transcription regulation (PubMed:8188694, PubMed:10817758, PubMed:11698476, PubMed:14718551, PubMed:18809583, PubMed:31358969). Predominantly acts as a RNA-binding protein: binds preferentially to the 5'-[CU]CUGCG-3' RNA motif and specifically recognizes mRNA transcripts modified by C5-methylcytosine (m5C) (PubMed:19561594, PubMed:31358969). Promotes mRNA stabilization: acts by binding to m5C-containing mRNAs and recruiting the mRNA stability maintainer ELAVL1, thereby preventing mRNA decay (PubMed:10817758, PubMed:11698476, PubMed:31358969). Component of the CRD-mediated complex that promotes MYC mRNA stability (PubMed:19029303). Contributes to the regulation of translation by modulating the interaction between the mRNA and eukaryotic initiation factors (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 (PubMed:27559612, PubMed:29073095). Probably sorts RNAs in exosomes by recognizing and binding C5-methylcytosine (m5C)-containing RNAs (PubMed:28341602, PubMed:29073095). 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 (PubMed:29712925). Also involved in pre-mRNA alternative splicing regulation: binds to splice sites in pre-mRNA and regulates splice site selection (PubMed:12604611). Also able to bind DNA: regulates transcription of the multidrug resistance gene MDR1 is enhanced in presence of the APEX1 acetylated form at 'Lys-6' and 'Lys-7' (PubMed:18809583). Binds to promoters that contain a Y-box (5'-CTGATTGGCCAA-3'), such as MDR1 and HLA class II genes (PubMed:8188694, PubMed:18809583). Promotes separation of DNA strands that contain mismatches or are modified by cisplatin (PubMed:14718551). Has endonucleolytic activity and can introduce nicks or breaks into double-stranded DNA, suggesting a role in DNA repair (PubMed:14718551). The secreted form acts as an extracellular mitogen and stimulates cell migration and proliferation (PubMed:19483673). Homodimer in the presence of ATP (PubMed:10817758, PubMed:11851341). Component of the coding region determinant (CRD)-mediated complex, composed of DHX9, HNRNPU, IGF2BP1, SYNCRIP and YBX1 (PubMed:19029303). Identified in a IGF2BP1-dependent mRNP granule complex containing untranslated mRNAs (PubMed:17289661). Component of the U11/U12 snRNPs that are part of the U12-type spliceosome (PubMed:15146077). Identified in a histone pre-mRNA complex, at least composed of ERI1, LSM11, SLBP, SNRPB, SYNCRIP and YBX1 (By similarity). Interacts with IGF2BP1 and RBBP6 (PubMed:17289661, PubMed:18851979). Component of cytoplasmic messenger ribonucleoprotein particles (mRNPs) (PubMed:19029303). Interacts with AKT1, MBNL1, SFRS9, SFRS12, ALYREF/THOC4, MSH2, XRCC5, WRN and NCL (PubMed:12604611, PubMed:14559993, PubMed:14718551, PubMed:15806160, PubMed:18335541). Interacts (via C-terminus) with APEX1 (via N-terminus); the interaction is increased with APEX1 acetylated at 'Lys-6' and 'Lys-7' (PubMed:18809583). Interacts with AGO1 and AGO2 (PubMed:17932509). Interacts with ANKRD2 (PubMed:15136035). Interacts with DERA (PubMed:25229427). Interacts with FMR1; this interaction occurs in association with polyribosome (By similarity). Interacts with ZBTB7B (By similarity). Interacts with HDGF (isoform 1) (PubMed:26845719). Interacts with ELAVL1; leading to ELAVL1 recruitment on C5-methylcytosine (m5C)-containing mRNAs and subsequent mRNA stability (PubMed:31358969). Predominantly cytoplasmic in proliferating cells (PubMed:12604611). Cytotoxic stress and DNA damage enhance translocation to the nucleus (PubMed:14718551). Localized in cytoplasmic mRNP granules containing untranslated mRNAs (PubMed:25229427). Shuttles between nucleus and cytoplasm (PubMed:25229427). Localized with DDX1, MBNL1 and TIAL1 in stress granules upon stress (PubMed:18335541). Secreted by mesangial and monocytic cells after inflammatory challenges (PubMed:19483673). In the CSD domain, Trp-65 specifically recognizes C5-methylcytosine (m5C) modification through its indole ring. Ubiquitinated by RBBP6; leading to a decrease of YBX1 transcactivational ability. In the absence of phosphorylation the protein is retained in the cytoplasm. Cleaved by a 20S proteasomal protease in response to agents that damage DNA. Cleavage takes place in the absence of ubiquitination and ATP. The resulting N-terminal fragment accumulates in the nucleus (By similarity). Belongs to the YBX1 family. Extended N-terminus.