Lymphocyte antigen 6D (Ly-6D), also known as thymocyte B-cell antigen (ThB), is a 16 kD GPI-linked cell surface glycoprotein. It belongs to the Ly6 gene family and contains one urokinase receptor (uPAR) /Ly6 domain. Ly-6D is involved in cell-cell adhesion and signal transduction. Ly-6D marks the earliest stage of B cell lineage commitment from the common lymphoid progenitors (CLP). It is expressed on B cells, immature thymocytes, and plasmacytoid dendritic cells.