The recently classified type III interferon group consists of three IFN-λ (lambda) molecules called IFN-λ1, IFN-λ2 and IFN-λ3 (also called IL29, IL28A and IL28B respectively). These IFNs signal through a receptor complex consisting of IL10R2 (also called CRF2-4) and IFNLR1 (also called CRF2-12). Recently, a new protein with a similar function related to IFN-λ3 was found around the same genomic locus and was designated IFN-λ4. Its intracellular signaling was through IFNLR1 and therefore thought to be a type III interferon. However, the evidence of its in vivo bioactivity is still debatable.
^Lu, YF; Goldstein, DB; Urban, TJ; Bradrick, SS (February 2015). "Interferon-λ4 is a cell-autonomous type III interferon associated with pre-treatment hepatitis C virus burden.". Virology476: 334–40. doi:10.1016/j.virol.2014.12.020. PMID25577150.