A death cross is a technical indicator that occurs when a stock's 50-day moving average crosses below its 200-day moving average. This is generally considered a bearish signal, suggesting that the stock's long-term trend may be changing. In the case of Illumina, the formation of a death cross could lead to a decline in the stock price in the short term. However, it's important to note that seasoned investors don't blindly trade death crosses and consider other factors like price levels and company fundamentals before making decisions.