Junctional modifications of T cell receptor (TcR) and immunoglobulin (Ig) gene joining regions provide great diversity to respective protein repertoires. The addition of non‐germ‐line‐encoded nucleotides (N‐regions) in the V‐Jγ junction is one such modification which is developmentally regulated, rarely evident in the fetal animal, but common in the adult. A question has recently arisen as to whether developmentally patterned N‐region additions in V‐Jγ joins are a reflection of T cell progenitors which are committed to particular types of rearrangement prior to the event, or of changing environmental influences on uncommitted cell populations. To address this question with regard to theVγ3‐Jγ1 join, T cells were examined in the fetal thymic organ culture (FTOC), a system with which the environment of early progenitor cells could be deliberately altered. At various times following FTOC initiation, cells were isolated for examination by the polymerase chain reaction, cloning and sequencing. Vγ3‐Jγl sequences within genomic DNA as well as cDNA were evaluated. Data from these studies revealed frequent N‐region additions within V‐Jγ joins among day 14 fetal thymocyte populations, a situation dissimilar from that
in vivo. Also dissimilar from the in vivosituation was the degree of exonuclease activity evident in FTOC. The canonical Vγ3‐Jγl join (a frequent junction lacking N‐region addition) was recognized in all experiments, but was least common among DNA versus cDNA sequences. Results illustrate that early progenitor cell populations are not programmed to exclude junctional modifications from Vγ3‐Jγ1 joins.