Given an abstract of a paper, generate a title for this paper such that conveys the key focus of the paper.

Abstract: T lymphocytes differentiate and mature in the thymus. It is here that thymocytes with reactivities to self antigens are eliminated and those with specificities to 'altered' self-major histocompatibility complex (MHC) gene products are positively selected. The selections are presumably carried out on the basis of their T-cell antigen receptors (TcR). The genes of the alpha- and beta-chain T-cell antigen receptors have been cloned. A third T-cell specific gene capable of undergoing somatic rearrangement has also been identified; the role of this third gene is not known. An order of expression of gamma, beta, then alpha is found during T-cell ontogeny. But although alpha- and beta-chain messages are often functional, gamma transcripts are rarely functional in thymocytes or mature T cells. To define the sequential order of expression of these genes further and to continue the search for a possible role for the TcR gamma gene products, we investigated the expression of 'functional' alpha-, beta- and gamma-chain transcripts in young athymic mice. These mice express an undetectable amount (less than one in 8 X 10(5) spleen messages) of 'full-length' alpha- and beta-chain T-cell receptor transcripts, but an increased level of expression of 'full-length' gamma chain messages. Nucleotide sequence analysis of four gamma complementary DNAs show that all four gamma transcripts sequenced are functional. These findings suggest that gamma gene products may be important in a prethymic or extrathymic pathway and may represent a second type of T-cell recognition, possibly in a lineage in which alpha and beta genes are not used.
Athymic mice express a high level of functional gamma-chain but greatly reduced levels of alpha- and beta-chain T-cell receptor messages.