Shrivastava et al., An orphan receptor tyrosine kinase family whose members serve
    as nonintegrin collagen receptors.
    Molecular Cell 1, 25–34 (1997).

    Gassmann et al., Aberrant neural and cardiac development in mice lacking the
    ErbB4 neuregulin receptor.
    Nature 378, 390-394 (1995).

   Lai and Lemke, An extended family of protein-tyrosine kinase
   genes differentially expressed in the vertebrate nervous system.
   Neuron 6, 691-704 (1991).