Mahoganoid (Mgrn1^md) arose spontaneously on strain C3H/HeJ at the Jackson Laboratory in 1957. The Igl-1^b allele at the immunoglobulin lamba chain cluster 1 locus is found in strains SJL, YBR, BSVS and others and the Igl1-r^lo allele occurs in strains SJL, BSVS and others. In 1972 Dr. M. C. Green received the fused phalanges (Fbn2^fp) stock from Dr. Katrina Hummel. Fused phalanges arose spontaneously in strain C3HeB/FeHu. The stock Dr. Green received had been crossed once to C57BL/6J and it was then brother x sister bred to F20 in 1976. At F18 the stock was given to Dr. Muriel Davisson and it was continued to be inbred to F21 when a fused phalanges male was outcrossed to a homozygous mahoganoid (Mgrn1^md) female. This mahoganoid female was the N2 offspring of a backcross that began with a mating of C57BL/6J x C3H/He-Mgrn1^md/+ from which an F1 was bred back to a C3H/HeJ-Mgrn1^md/Mgrn1^md, which was at F69. The N2 homozygous female resulting from this breeding was bred with the F21 fused phalanges male and their offspring were bred brother x sister to F20. At that point, a mahoganoid male was outcrossed to an Lyst^bg homozygous female on a congenic SJL/J background at generation N12F4 from Dr. John Roths. The resulting stock was sibling mated and a homozygous mahoganoid female was backcrossed to an SJL/J male. The stock was then bred as homozygous for mahoganoid (Mgrn1^md),Igl-1^b and Igl1-r^lo all on chromosome 16. It was cryopreserved in 1991 by mating homozygotes at F27 and F28.