The mK3 cell line represents early metanephric mesenchyme prior to induction by the ureteric bud. These cells show a spindle-shaped, fibroblast morphology. They express genes characteristic of early mesenchyme, including Hoxa 11, Hoxd 11, collagen I, and vimentin. Moreover, the mK3 cells display biological function associated with early metanephric mesenchyme. In organ co-culture experiments these cells can induce growth and branching of the ureteric bud.