Looking into the current session, Illumina Inc. ILMN is trading at $311.85, after a 3.29% decrease. Over the past month, the stock decreased by 12.06%, but over the past year, it actually increased by 0.73%. With questionable short-term performance like this, and great long-term performance, long-term shareholders might want to start looking into the company's price-to-earnings ratio.

Assuming that all other factors are held constant, this could present itself as an opportunity for shareholders trying to capitalize on the higher share price. The stock is currently under from its 52 week high by 22.85%.

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The P/E ratio measures the current share price to the company's EPS. It is used by long-term investors to analyze the company’s current performance against its past earnings, historical data and aggregate market data for the industry or the indices, such as S&P 500. A higher P/E indicates that investors expect the company to perform better in the future, and the stock is probably overvalued, but not necessarily. It also shows that investors are willing to pay a higher share price currently, because they expect the company to perform better in the upcoming quarters. This leads investors to also remain optimistic about rising dividends in the future.

Depending on the particular phase of a business cycle, some industries will perform better than others.

Illumina Inc. has a lower P/E than the aggregate P/E of 79.25 of the Diagnostics & Research industry. Ideally, one might believe that they might perform worse than its peers, but it’s also probable that the stock is undervalued.

P/E ratio is not always a great indicator of the company's performance. Depending on the earnings makeup of a company, investors may not be able to attain key insights from trailing earnings.