Elan Corporation might be going from strength to strength as it repositions itself from being a drug delivery company to a focused pharmaceuticals developer and manufacturer, but for most of the institutions operating in the Irish stock market, Elan's phenomenal growth is becoming more and more of a problem.

Institutions that want to track the ISEQ index are faced with a situation where Elan makes up a quarter of it - and by the time it issues 35 million new shares to Dura shareholders, it will account for close on 30 per cent of the index.

There is simply no way that ISEQ-investing institutions can match Elan's 25 per cent plus weighting. And as a result, some institutions have taken to remodelling the ISEQ to exclude the Elan influence.

Most of the Irish institutions belatedly discovered Elan as an investment after spurning the stock for years. Notoriously riskaverse Irish fund managers simply refused to invest in a company that at the time was a tiddler in a sector that they didn't understand and on which there was little independent research.

That's all changed now. Elan is no longer a tiddler and there's reams of research bumf produced by Irish and international brokers. But it's now too late for Irish institutions to get a decent chunk of Elan stock. Now 80 per cent of Elan shares are held by US investors who are jealously guarding the stock they have.