Glucosinolates are a group of glycosides stored within cell vacuoles of all cruciferous vegetables. An enzyme called myrosinase found in plant cells catalyzes glycosides to a variety of products including isothiocyanates and indoles.
Because cooking cruciferous vegetables greatly reduces the levels of glucosinolates in foods, it is important to enjoy them raw. And if you cook them, opt to lightly steam or quickly sauté to achieve the most health benefits.
Verhoeven, D.T.H., Goldbohm, R.A., van Poppel, G., Verhagen, H., and van den Brandt, P.A. 1996. Epidemiological studies on brassica vegetables and cancer risk. Cancer Epidemiol. Biomarkers Prev. 5: 733-748.