Two alternatively spliced mouse lymphocyte and brain ltk cDNAs predict small transmembrane tyrosine kinases that use CUG translational start codons and that differ upstream of their transmembrane segment. A recently isolated human neuroblastoma ltk cDNA, in contrast, includes a regular AUG start codon and predicts a more conventional receptor kinase with a larger N-terminal segment. This raised the suggestion that previous mouse cDNAs may have been aberrantly spliced or incomplete and questioned the significance of a recent study that localized the lymphoid ltk protein to the endoplasmic reticulum. Here we show that mice tissue-specifically express four ltk mRNAs. In addition to the two previously described lymphoid and brain mRNAs, we now describe two mRNAs from C1300 neuroblastoma cells that start with five exons which are absent from lymphoid or brain transcripts. The pair of C1300 mRNAs differ by the same alternatively spliced exon that distinguishes brain from lymphoid mRNAs and predict much larger receptor-type kinases that use regular AUG start codons. Our results also show that at least one of the larger, more conventional C1300 ltk receptors shares the endoplasmic reticulum localization of the shorter lymphoid protein.