Interleukin 17 Receptor D (IL-17RD), also known as Sef (similar expression to FGFs), is a type I transmembrane IL-17R-related protein that can bind to FGF receptors 1 and 2 and antagonize ERK signaling by preventing translocation to the nucleus. The full-length 739aa hSef-a isoform is a 120kD cell surface glycoprotein expressed in most tissues (except thyroid). Three other isoforms have alternate N-termini shortened by 24, 32 or 144aa. Isoform hSef-b is an 80kD non-glycosylated intracellular Golgi membrane protein within thyroid, testes, pituitary and fetal brain. Human IL-17RD shows 90% aa identity with mouse IL-17RD in the extracellular/lumenal region.