In this week's Nature there is some exciting news about what makes brown adipose tissue, or "brown fat". It turns out that brown fat actually comes from muscle precursor cells, not fat cells, and bone morphogenic protein 7 (Bmp7) can induce the transformation.
Brown fat is very important in hibernation as it can generate heat by short-circuiting the mitochondrial proton pump. At very low temperatures, animals cannot shiver so they need other heat generating mechanisms. Its also important for infant mammals, including humans.
Bmp7 is part of the TGF-beta superfamily (transforming growth factors). In the human its on chromosome 20. 7 exons, very typical gene in that respect.
Brown fat has generated a lot of controversy over the years (since 1551!); this discovery seems like a big step forward.