In this study, we describe the molecular cloning and characterization of a Src-like adaptor protein gene embedded within the genomic organization of the human thyroglobulin (Tg) gene. This gene was identified by exon trapping on overlapping cosmids encompassing the largest Tg intron. A 2.6-kb transcript, with the highest levels of expression in fetal brain and lung, was detected on Northern blots. Two full-length cDNAs (one alternatively spliced) were isolated from a fetal brain library, both containing an open reading frame of 276 amino acids, but lacking a catalytic tyrosine kinase domain. The gene shows a high degree of cross-species similarity and appears to be transcribed in the direction opposite to Tg. This gene, designated hslap, appears to be the human ortholog of the recently described gene for the murine Src-like adaptor protein (mSLAP), a candidate intermediate in the signal-transduction pathway of the Eck receptor tyrosine kinase. Human slap is located in the candidate region for a recessive demyelinating neuropathy on chromosome 8q24, but sequence analysis failed to identify mutations, suggesting that it is not the gene for this disease.