The London-listed group went public with its approach after the U.S. firm turned it down. Shire (SHPG) said its unsolicited offer of 0.1687 Shire American depositary receipts per share valued each Baxalta share at $45.23. Shares in Baxalta (BXLT) jumped following the news. The Illinois-based firm, which has a staff of around 16,000, develops biotech treatments for rare blood conditions, cancers and immune system disorders. It had proforma revenue of $6 billion in 2014. The move is the latest in a wave of mergers and acquisitions in the healthcare sector since the start of 2014, stretching from large drugmakers buying up smaller rivals to consolidation among makers of generic medicines and tie-ups between insurers. Shire's all-share offer, which represents a premium of 36 percent over Baxalta's stock price on Aug. 3, would give the U.S. firm's shareholders about 37 percent of the combined group.