SECTM1 (secreted and transmembrane 1), also called K12, is either found as an ~27kD intracellular type I transmembrane protein that shows a perinuclear, Golgi-like staining pattern, or as a 20kD soluble, secreted form. SECTM1 protein expression is detected in some myeloid cells, such as stimulated monocytes, immature monocyte-derived dendritic cells, and granulocytes. In the thymus, it is also expressed by epithelia and fibroblasts. Stimulation with IFNg is often necessary to detect SECTM1 expression, and it is thought to be an interferon earlyresponse gene. Human SECTM1 cDNA encodes 248aa, including a 28aa signal sequence, a 117aa extracellular domain (ECD) with one potential N-linked glycosylation site, a 21aa transmembrane sequence, and an 82aa cytoplasmic sequence. The ECD shows some similarity to Ig domains. The human SECTM1 ECD shares limited (~45-51%) aa sequence identity with mouse SECTM1 (a or b) and rat SECTM1 ECD. SECTM1 from human and mouse each show species specific binding of CD7 and co-stimulate CD4+ and CD8+ T cells through CD7, either alone or through synergy with CD28. Immobilized recombinant human SECTM1/Fc also enhances human NK cell expression of activation markers such as CD25, CD69 and CD54.