Updated from 8:13 a.m. EDT

Monday's marriage of Biogen ( BGEN) and Idec Pharmaceuticals ( IDPH) combines two remarkably similar biotech portfolios, each containing one reliable blockbuster and a shaky newcomer.

Idec's core strength is its oncology portfolio, including the non-Hodgkin's lymphoma treatment Rituxan, which did $1.5 billion of worldwide sales last year, proceeds that were shared with Genentech ( DNA). It also recently launched the cancer treatment Zevalen. Biogen's blockbuster is the multiple sclerosis treatment Avonex, which did $1.1 billion of sales last year, while its newcomer is the psoriasis drug Amevive.

"When you combine these companies, you get larger scale, less dependence on a single marketed product, more diversification and less risk from any individual pipeline project," noted Adam Feuerstein, a columnist for TheStreet.com's sister site, RealMoney.com. "All solid reasons for a merger, but not exactly sexy reasons. But then, does every deal have to be sexy?"

The $6.9 billion stock swap, in which Biogen holders will exchange their stock for 1.15 Idec shares, will leave Idec shareholders with 50.5% of the combined company. The implied value of $44.82 worth of Idec shares was a slight premium to Biogen's Friday close of $43.80.

After initially rising on the deal, Biogen shares were recently down $1.50, or 3.4%, to $42.30. Tracing a similar arc, Idec was recently down $1.54, or 4%, to $37.43.

The companies estimated combined pro forma revenue at $1.5 billion and put their combined research and development budget at more than $550 million, with 1,000 employees devoted to R&D. Their pipeline includes Antegren, now in phase III trials for multiple sclerosis and Crohn's disease; and Rituxan for the additional treatment of rheumatoid arthritis and other autoimmune diseases, also in phase III studies.

Separately, Biogen lowered its second-quarter earnings estimates, citing lower-than-expected sales of its hepatitis drug Itron. Biogen forecast earnings of 32 cents to 38 cents a share, compared with the 42-cent consensus estimate. It said its old forecast of 2003 earnings of $1.72 to $1.85 a share remains achievable.

Based on expected growth from the combined product lineup, the contribution of pipeline products, and anticipated sales, marketing and manufacturing synergies, the companies expect the combined Biogen Idec to achieve 15% compound annual revenue growth, and roughly 20% compound annual cash earnings per share growth between 2003 and 2007.

Genentech, which because of the Rituxan partnership was often mentioned as a possible Biogen buyer, was recently down $1.75, or 2.4%, to $72.57.