CD44 and breast cancer: For example, in colorectal cancer, CD44 is broadly expressed across both CSC and non-CSC populations, leading to inconsistent results in CSC isolation.138 Similarly, the widely used CD44+/CD24− phenotype in breast cancer does not consistently correlate with tumorigenic capacity across all subtypes.139 These limitations suggest that while isoform-specific expression (e.g., CD44v4-10, v6, v8-10) may offer improved specificity, CD44 should ideally be used in combination with other markers or functional assays to accurately identify CSCs in a tumor type-dependent manner.