AbstractThe t(4; II )(q21;q23) has been associated with acute lymphocytic leukemia (ALL) especially in infants. The t(4; II) breakpoint on chromosome II is cytogenetically indistinguishable from breakpoints for other leukemia‐associated translations affecting 11q23. The molecular basis of the t(4;lI) is unknown although a number of genes have been mapped to IIq23. TheCD3D, G, andEgenes have been positioned proximal to the IIq23 breakpoint of the 4; II translocation while theTHY IandETSIgenes have been mapped distal to this breakpoint. We report evidence thatCD3Gis within 200 kb of the 4; II breakpoint as observed by pulsed field gel analysis. A rearrangement of theCD3Ggene has been observed in a cell line derived from a patient with the t(4;II) translocation and in a hybrid cell line containing the derivative IIq chromosome derived from this cell line, using the restriction enzymes Sacll andClal. Similar rearrangements usingSacllwere observed in 2 further patients with ALL and the t(4; II) translocation. No rearrangements in the same DNA were observed usingETSI, THY I, andDl IS29and a range of rare cutter restriction enzymes.CD3Gthus provides a tool for the cloning and analysis of the 4; II trasnslocation, and poses a question of its possible involvement at long range with this translocatio