MicroRNA, siRNA, stRNA, shRNA, ncRNA, RNAi ... those of us who haven't been paying close attention to the expanding field of RNA interference might find ourselves struggling in this sea of letters. Two articles in this issue go some way towards providing a lifeline. Lin He and Greg Hannon (page 522) describe the origin and function of microRNAs, whereas Jonathan Hall (page 552) discusses how to design more potent and specific siRNAs.

This issue also contains a special focus on Organogenesis, a follow-up to our 2002 Organogenesis special issue (http://www.nature.com/nrg/focus/organogen/), which featured articles on early liver specification, pancreatic stem-cell identity, heart patterning, tubulogenesis and kidney development. This focus covers the development of three quite different parts of the mammalian anatomy — teeth (page 499), ears (page 489) and reproductive organs (page 509).

Comparative studies have contributed greatly to our knowledge of the development of these three systems, as have advances in genetic techniques and other technological innovations, such as expression profiling. Developmental biologists can now deconstruct organogenesis into individual steps, each of which involves genetic changes that control not only cell identity, but also other cellular properties, such as shape, adhesion and polarity.

A special web focus to accompany the 2004 Organogenesis issue (http://www.nature.com/nrg/focus/organogen2) will appear on 20th July, to coincide with this year's Society for Developmental Biology meeting (http://sdb.bio.purdue.edu/PDFs/2004.sdb.call.pdf). The reviews in the special focus will be complemented by a selection of relevant articles from other journals in the Nature Publishing Group.