Signet Jewelers (SIG) shares were over 7% higher in recent premarket trading Friday after shareholder Select Equity Group said in a regulatory filing it has amassed around 4.2 million Signet Jewelers shares, approximately 9.7% of the company's total equity, for $153 million.

Select Equity said in a letter accompanying the filing that it believes the shares are undervalued and is urging the company's board to explore strategic options including a potential sale.