Biogen Idec Inc.

Spark Therapeutics Inc. hit a $1.2 billion valuation on the biotechnology firm’s first day on the market, reflecting growing investor enthusiasm for the once-beleaguered field of gene therapy, a field that has produced the first $1 million drug.  The treatments focus on diseases where a single mutation in the genetic code is known to be the cause. Fix or replace the genetic code, and, …