However, as I'm sure you realize, but what may need to be pointed out is,
though these(biotechs)all trade with the sector, day to day, the best ones(e.g.Ariad) progressively move into higher relative orbits within those sectors, as fundamentals improve, over time.
So if you're not trading a lot, day to day(unless we get the big pan sector crash), there's really no need too be to concerned.
Of course, this has been true for a long time now, and, IMO, is not likely to change for the foreseeable future.