Heterogeneous nuclear ribonucleoprotein A1 is a protein that in humans is encoded by the HNRNPA1 gene. Mutations in hnRNP A1 are causative of amyotrophic lateral sclerosis and the syndrome multisystem proteinopathy. Function This gene belongs to the A/B subfamily of ubiquitously expressed heterogeneous nuclear ribonucleoproteins (hnRNPs). The hnRNPs are RNA binding proteins and they complex with heterogeneous nuclear RNA (hnRNA). These proteins are associated with pre-mRNAs in the nucleus and appear to influence pre-mRNA processing and other aspects of mRNA metabolism and transport. While all of the hnRNPs are present in the nucleus, some seem to shuttle between the nucleus and the cytoplasm. The hnRNP proteins have distinct nucleic acid binding properties. The protein encoded by this gene has two repeats of quasi-RRM domains that bind to RNAs in the N-terminal domain which are pivotal for RNA specificity and binding. The protein also has a glycine rich arginine-glycine-glycine (RGG) region called the RGG box which enables protein and RNA binding. It affects many critical genes that are responsible for controlling metabolic pathways at the transcriptional, post-transcriptional, translation, and post-translation levels. It is one of the most abundant core proteins of hnRNP complexes and it is localized to the nucleoplasm. This protein, along with other hnRNP proteins, is exported from the nucleus, probably bound to mRNA, and is immediately re-imported. Its M9 nuclear localisation sequence (NLS), a glycine rich region downstream from the RGG box, acts as both a nuclear localization and nuclear export signal. The encoded protein is involved in the packaging of pre-mRNA into hnRNP particles, transport of poly A+ mRNA from the nucleus to the cytoplasm, and may modulate splice site selection. Multiple alternatively spliced transcript variants have been found for this gene but only two transcripts are fully described. These variants have multiple alternative transcription initiation sites and multiple polyA sites. Post translational modifications are also known to affect hnRNP A1's function. Methylation of arginine residues in the RGG box may regulate RNA-binding activity. Kinases such as protein kinase C (PKC), mitogen-activated protein kinases (MAPKs), and ribosomal S6 kinases (S6Ks) phosphorylate serine residues at both the N and C terminals to regulate function. Phosphorylation of the C-terminal region causes cytoplasmic accumulation of the protein. However, addition of O-GlcNAcylation (GlcNAc) moiety to serine or threonince is a common and reversible modification that impairs the protein's binding of karyopherin beta (Transportin-1) resulting in nuclear localization of hnRNPA1. Interactions hnRNP