A 2.5 kilobase (kb) cDNA clone containing 92% of the coding region for human transmembrane secretory component (SC) or poly-Ig receptor, was isolated from a mammary gland cDNA library. The cDNA clone encoded a protein of 693 amino acids which showed 99% homology with the primary amino acid sequence of human free SC as reported by Eiffert et al. (1), and 54% homology with the deduced amino acid sequence of rabbit transmembrane SC for which cDNA was cloned by Mostov et al. (2). Northern blot analysis showed mRNA expression in various human exocrine tissues in good agreement with our previous immunohistochemical studies of SC.