Mice transgenic for a hybrid gene containing the liver promoter of the mouse α-amylase gene (Amy-1a) fused to the SV40 tumor antigen coding region unexpectedly developed malignant brown adipose tissue tumors (malignant hibernomas). Expression of the α-amylase gene had previously been thought to be confined to the liver, parotid, and pancreas; however, analysis of white and brown adipose tissue from nontransgenic mice revealed expression of the endogenous Amy-1a gene in these tissues. Gene constructs driven by the Amy-1a liver promoter thus provide a means of targeting gene expression to the adipocyte cell lineage in transgenic mice. Moreover, the high frequency of metastases in the liver, lungs, spleen, heart, and adrenals of these mice provides an experimental system in which to study the development of disseminated malignancy.