LONDON (AP) — Renewed weakness in the tech-heavy Nasdaq index in the U.S. roiled global markets Friday.

The Nasdaq index fell a further 0.6 percent to 4,029 after slumping 3 percent on Thursday — its worst day since 2011 — as tech and biotech stocks took another hammering after a two-day recovery. The Nasdaq has been the main driver of global markets since last Friday when concerns over the valuation of many of its constituent stocks first erupted.