E V I D E N C E FOR THE LINKAGE OF' THE IGCu LOCUS TO A GENE CONTROLLING T H E I D I O T Y P I C SPECIFICITY OF ANTI-p-AZOPHENYLARSONATE ANTIBODIES IN STRAIN A MICE*

Rabbit anti-idiots~ic antisera specific for anti-p-azophenylarsonate (antiAr) t antibodies, raised in an individual strain A / J mouse, recognize determinants on anti-Ar antibodies induced in all members of this strain so far tested. These determinants have been found only on anti-At antibodies and not on normal imnmnoglobulins or on other anti-hapten or anti-protein antibodies induced in this strain (1-3). 2 The determinants on A/J anti-Ar antibodies are thus idiotypic and cross-reactive within the strain. In contrast, anti-Ar antibodies induced in most but not all other strains of mice (1, 3; Pawlak and Nisonoff, manuscript in preparation), lack the cross-reactive A/J idiotypic determinant. For this reason the idiotype of A/J anti-Ar antibody is potentially a genetic marker (phenotype). We report here finding the A/J idiotype on anti-Ar antibodies induced in strain AL/N mice; this strain was derived from an outcrossed strain A mouse (4). Strains AL/N and A/J share a number of characteristics, including similar H-2 complex loci (4) and most but not all IgCH allotypic determinants (5, 6), which are localized to the Fc fragment of the molecule. Strain AL/N then probably shares many genes with strain A/J but, according to Staats (4) cannot be considered a true subline of strain A (4). The presence of the cross-reactive idiotypic determinants on anti-Ar antibodies induced in strain AL/N mice is particularly fortuitous because we have recently completed, at the National Institutes of Health, the development of a congenic strain of BALB/c mice, AL/N IgCm in which the IgCH complex locus from AL/N mice was introgressively backcrossed onto BALB/c. This