2F1ab

2F1ab is not a standard gene designation in contemporary virology and often appears as a mislabeling or variant spelling of ORF1ab, the large replicase polyprotein region found in coronaviruses. In coronaviruses, ORF1ab is located at the 5' end of the genome and encodes a polyprotein that is cleaved into 16 non-structural proteins (nsp1–nsp16), which assemble into the replication-transcription complex essential for viral genome replication and transcription.

The extended ORF1ab product is produced through a ribosomal -1 frameshift during translation of ORF1a and ORF1b, yielding the pp1ab polyprotein. This polyprotein is subsequently cleaved by viral proteases into the mature nsps, including key enzymes such as nsp12 (RNA-dependent RNA polymerase), nsp13 (helicase), and the cysteine proteases nsp3 (papain-like protease) and nsp5 (3C-like protease). The nsps coordinate replication of the viral genome and the transcription of subgenomic RNAs that encode structural and accessory proteins.

Functionally, the ORF1ab-encoded nsps form the core of the replication-transcription complex, interact with membrane-associated structures in infected cells, and are central to replication fidelity and viral propagation. Because of its central role, ORF1ab is a common target in genomic annotations and diagnostic assays, and its sequence is frequently used in phylogenetic analyses of coronavirus lineages.

If encountered in literature or datasets, 2F1ab should be treated as nonstandard shorthand and cross-checked against established ORF1ab coordinates and annotations to avoid confusion. See also ORF1ab, non-structural proteins, and the SARS-CoV-2 genome.