They are classified according to their substrate specificity, sequence similarity, and domain organization into six groups: collagenases (MMP-1, MMP8, MMP-13, and MMP-18), gelatinases (MMP-2, MMP9), stromelysins (MMP-3, MMP-10), matrilysins (MMP7, MMP-26), membrane-type MMPs (MMP-14, MMP-15, MMP-16, MMP-24, MMP-17, and MMP-25), and other MMPs (MMP-12, MMP-19, MMP-20, MMP-21
, MMP-23, MMP-27, and MMP-28) .
Other MMPs that are not classified in previous categories include MMP-12, MMP-19, MMP-20, MMP-21
, MMP-23, MMP-27, and MMP-28 [6, 10, 11],
Retinoid receptor binding sites have been identified in the promoter region for MMP-21 expression, thus retinoic acid upregulates MMP21 mRNA in monocytic U937 cells  and the enzyme has been found in cancer cells.
In HaCaT cells, [10.sup.-6] and [10.sup.-7]M retinoic acid (RA) induced an upregulation of MMP-21 by 2.2-fold mRNA and protein, but this was not associated with cell proliferation or apoptosis.
In summary, the authors conclude that MMP-21 is expressed by suprabasal keratinocytes in certain benign disorders and is induced in differentiating keratinocytes, thus it may be an important protease in this process.
The present paper deals with a recently cloned member of the group, MMP-21, which has been implicated in fetal development, stromal remodelling, inflammation and cancer, but its physiological substrate(s) has not been identified.
A range of agents were tested for their ability to induce MMP-21 mRNA but only all-trans-retinoic acid ([10.sup.-6]M) and a protein kinase inhibitor called staurosporine were effective whereas tumour promoters, hormones, vitamin [D.sub.3] and dexamethasone had no effect.