What is NCBI and what databases does it provide?

NCBI (National Center for Biotechnology Information) is a major repository of biological data maintained by the US National Institutes of Health. Key databases include: GenBank (nucleotide sequences), RefSeq (curated reference sequences), PubMed (biomedical literature), Protein (protein sequences), Gene (gene-centric information), dbSNP (single nucleotide polymorphisms), SRA (Sequence Read Archive for raw sequencing data), PDB (protein structures linked from RCSB), and ClinVar (clinical variants). NCBI provides free access to these interconnected databases along with analysis tools like BLAST.