Insertion of multiple copies of a transgene containing a mouse mammary tumor virus long terminal repeat linked to a myc gene disrupted the gene. The insertion is accompanied by a small genomic deletion (~1-1.5 kb), and is located distal to cytogenetic band C1, near the 3' end of the gene in the intron preceding exon 20. This allele disrupts a regulatory element that affects the expression of the downstream Grem1 gene. This mutation was determined through complementation analyses to be allelic with Fmn1ld-TgBri137, Grem1ld, and Grem1ld-J. (J:1741, J:8077, J:38417)