GlaxoSmithKline plc (LSE/NYSE: GSK) and Sierra Oncology, Inc (Nasdaq: SRRA) announced that the companies have entered into an agreement under which GSK will acquire Sierra Oncology, a California-based, late-stage biopharmaceutical company focused on targeted therapies for the treatment of rare forms of cancer, for $55 per share of common stock in cash representing an approximate total equity value of $1.9 billion (£1.5 billion).