Abstract: The present invention provides a method of producing non-haematopoietic cells, in particular cells having one or more properties of endothelial cells, from monocytes. A range of diagnostic and prognostic assay methods for determining whether or not a subject has atherosclerosis or vascular complication associated therewith, or a predisposition for developing atherosclerosis or complication associated therewith employing this method are provided. A range of screens to identify and isolate compositions of matter that modulate the ability of monocytes to differentiate into such non-haematopoietic cells and/or that modulate the ability of such non-haematopoietic cells to become incorporated into endothelium e.g., vascular endothelium, are also provided.