Our recent studies suggested that the ret proto-oncogene protein is a cell surface receptor with a tyrosine kinase domain. However, the sequence of its cDNA lacked an amino-terminal signal sequence characteristic of membrane proteins. In the present study we have isolated and sequenced an additional cDNA clone. The nucleotide sequence of the cDNA obtained indicated that the 5' terminal 330 nucleotides differs from the published sequence of the ret proto-oncogene. In addition, the sequence contains an open reading frame with an amino-terminal signal sequence.