The phrase 'lie at the heart of something' suggests that a particular element is central to a broader situation or concept. Synonyms can include 'form the core,' 'constitute the essence,' 'occupy a central role,' 'be pivotal to,' and 'represent the foundation of.' Other expressions that are often used in place of this phrase include 'be the crux of the matter,' 'underpin,' 'be the centerpiece of,' and 'be integrally connected to.' These terms all communicate the significance of a particular feature or characteristic in relation to a larger whole, emphasizing its essential role and importance. Choosing the right synonym will help you convey your meaning with precision and clarity.