CD90.1, also known as Thy1.1, is a glycoprotein and member of the immunoglobulin superfamily (IgSF) that serves as a critical cell surface marker with broad applications in stem cell biology, immunology, and neuroscience. Initially characterized as a T cell marker, subsequent studies have demonstrated its widespread expression across diverse cell lineages, including neurons, fibroblasts, stem cells (e.g., hepatic progenitor cells), and specific immune cell subsets.