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290 G.M. Zinser et al.

is currently undefined, and further studies are needed before guidelines or require-
ments for human populations can be established. Collectively, studies to date have
confirmed that multiple components of the vitamin D signaling system are present
in normal mammary epithelial cells, but have also emphasized the need for addi-
tional research on regulation and function of these proteins in intact mammary tissue
in vivo, particularly in relation to maintenance of the differentiated phenotype.
In addition to gaps in knowledge of vitamin D signaling in mammary epithelial
cells, little is known about the in vivo compartmentalization of the metabolic
enzymes, transport proteins and receptor for vitamin D in the gland. Still unre-
solved as well are the molecular mechanisms for cellular uptake, storage and intra-
cellular transport of the various vitamin D metabolites in mammary tissue. Use of
targeted mouse models with cell type specific ablation of VDR, Cyp27B1 and other
candidate genes involved in vitamin D signaling should be highly informative in
clarifying some of the relationships postulated in our working model.

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