[The PTC1 chimeric oncogene (RET/PTC1) is generated by fusion of the RET proto-oncogene receptor tyrosine kinase domain to the 5-prime terminal region of the H4 gene containing a putative leucine zipper. H4 shows no significant homology to known genes and the protein function is unknown. The RET receptor ligand is Glial Cell Line-Derived Neurotrophic Factor. The putative H4 leucine zipper mediates dimerization of the PTC1 oncoprotein, essential for PTC1 transforming activity. (from OMIM 601985 and NCI) ( NCI )]