What is the difference between nr and refseq? Based on NCBI's own definition, "RefSeq database is a non-redundant set of reference standards derived from the INSDC databases that includes chromosomes, complete genomic molecules (organelle genomes, viruses, plasmids), intermediate assembled genomic contigs, curated genomic regions, mRNAs, RNAs, and proteins. ", refseq is also redundant. But when you perform blast searches, you can select either nr/nt or refseq. So I assume there is a difference.