While the Underweight rating suggests a cautious view on Illumina's stock, the increase in price target from $85 to $100 indicates a positive adjustment in valuation expectations by Barclays. This could reflect an improved outlook on the company's future performance or market conditions, yet the Underweight rating still signals that Barclays believes the stock may underperform the broader market or its sector. Investors should consider this mixed signal when evaluating Illumina's stock.