The present invention relates to MDK1 polypeptides, nucleic acids encoding such polypeptides, cells, tissues and animals containing such nucleic acids, antibodies to such polypeptides, assays utilizing such polypeptides, and methods relating to all of the foregoing. Methods for treatment, diagnosis, and screening are provided for diseases or conditions characterized by an abnormality in a signal transduction disorder. The signal transduction pathway involves an interaction between a MDK1 receptor tyrosine kinase and a receptor for the kinase. The MDK1 receptor tyrosine kinase may be truncated and lack a kinase domain and may be selected from the group consisting of MDK1.T1, MDK1.T2, MDK1.1 and MDK1.2.