Abstract: The present invention relates generally to therapeutic and diagnostic agents. More particularly, the present invention provides molecules having structural features characteristic of immunoregulatory signalling (IRS) molecules and which are expressed by cells of haematopoietic lineages such as, in particular, leukocytes. The molecules of the present invention find broad application inter alia as diagnostic markers for cells, targets for cell therapy and as validated drug targets in order to modulate the immune response and to treat, prevent and diagnose a range of diseases conditions including cancer, genetic disease, inflammatory conditions and conditions associated with aberrant haematopoietic cell function or activity. The present invention extends to binding partners of the instant molecules such as, for example, antibodies, ligands, adaptor and other signalling associated molecules, agonists and antagonists and to methods of screening for same.