Dwarfism gene under scrutiny.

Geneticists are finally finding the genes responsible for most cases of dwarfism. Last year, two groups reported that achondroplasia, the most common genetic form of dwarfism, results from a small mutation in the gene that contains the instructions for fibroblast growth factor receptor 3 (FGFR3), a protein needed for skeletal development in the embryo.

They also showed that in almost all cases of achondroplasia, the mutation consists of an alteration of exactly the same nucleotide, one of DNA's basic building blocks.

This nucleotide's mutation rate--how often it spontaneously changes--is the highest ever calculated, notes Clair A. Francomano of the National Center for Human Genome Research in Bethesda, Md. "This is a mutation that keeps occurring again and again," she says. "I can't begin to give you an

explanation as to why."

This year, in the July Nature Genetics, Francomano and her colleagues reported that a less severe form of dwarfism, hypochondroplasia, also stems from a mutation on the FGFR3 gene, but at a completely different nucleotide. Among the 30 hypochondroplasia patients they've studied, 29 have a mutation at that particular nucleotide, she says.

One benefit of the new data on FGFR3, says Francomano, is that she can offer prenatal testing to couples who are both achondroplasic. That's important, because if each parent passes on the mutated FGFR3 gene, their child will have no normal copy. Babies with two copies of the mutated FGFR3 gene don't survive beyond a few months because of their severe skeletal abnormalities, she explains.