Does an Indian cancer biology researcher need to have identified a specific niche before applying for PhD programs in the US or Singapore?

Not having a defined niche at the point of application is completely normal and is not a disadvantage in the US PhD system. Most US PhD programs, particularly at research universities, operate on a rotational model during the first year, where students rotate through multiple labs before committing to a specific advisor and research direction. This is deliberately built into the program structure precisely because most applicants do not yet know exactly where their research interests will settle.

• What matters for the application is demonstrating genuine intellectual curiosity, a strong academic record, research experience that shows you can work in a lab environment, and a convincing narrative about why you want to do a PhD and what broad area you are drawn to.
• For cancer biology specifically, having research experience in any related area, cell biology, molecular biology, genomics, computational biology, is more important than having a specific hypothesis you want to pursue.
• The research direction will refine itself during the program; what the application needs to show is that you have the aptitude and motivation to pursue it.