Here are some suggestions:

• Check your spelling.
• If you searched for an author name using initials, try again using only the author's last name.
• Truncate words by using the asterisk (*) to find word variations: embryo* retrieves embryo, embryos, embryonic.
• Pay particular attention to terms containing hyphens, periods, or other punctuation. Punctuation is ignored and letters or numbers separated by punctuation are treated as separate words: Search IL-4 with il-4 or il 4 (with a space), but not with il4 (without a space). Search mRNA with mrna, not with m-rna or m rna.
• Avoid Greek letters, superscripts, subscripts, and other special characters.
• To search for an author name containing special characters, use the wildcard * to truncate the author name before the special character. For example, to search for articles by Christophe Fermé, search for Ferm*.