IFIT1B (interferon-induced protein with tetratricopeptide repeats 1B) is an interferon-stimulated gene encoding an RNA-binding protein with limited antiviral capacity compared to its paralog IFIT1. Unlike IFIT1, which selectively inhibits viruses lacking 2'-O-methylation at their 5' cap, IFIT1B diverged from IFIT1 over 100 million years ago and exhibits distinct functional properties 1. IFIT1B preferentially binds cap0-RNA with high affinity (~9-23 nM) but retains reduced binding to cap1-RNA (~450 nM), whereas IFIT1 cannot bind methylated cap1-RNA 2. In murine systems, Ifit1b binds preferentially to cap1-RNA and can restrict mouse coronavirus translation when complexed with Ifit1c 3. IFIT1B functions through competitive inhibition of the eIF4F translation initiation complex at the 5' cap region 2. Clinically, IFIT1B shows prognostic significance in lung adenocarcinoma as part of an RNA-binding protein signature predicting patient survival 4. IFIT1B expression is downregulated following hepatitis C virus clearance in HIV/HCV-coinfected patients, correlating with improved liver disease markers 5. Additionally, IFIT1B serves as a biomarker predicting TNF inhibitor response in rheumatoid arthritis, with combinations including IFIT1B achieving 94% predictive accuracy 6.