Culture medium

The invention relates to improved culture methods for expanding epithelial stem cells in a media comprising an ErbB3/4 ligand, one or more FGFR2b ligands and a BMP inhibitor, wherein the ErbB3/4 ligand induces the heterodimerization of ErbB3 or ErbB4 with ErbB2, wherein the FGFR2b ligand elicits tyrosine phosphorylation of FRS2α or FRS2β, and wherein the BMP inhibitor binds to a BMP molecule to form a complex wherein the BMP activity is neutralized or acts as an antagonist or reverse agonist. The invention further relates to obtaining lung organoids using the said method, wherein the lung organoids have been passaged for more than 3 passages. Uses of said lung organoids in drug discovery is also included.