Human milk oligosaccharides (HMOs) are a family of over 200 diverse sugars found in human breast milk. These sugars bring numerous health benefits to infants: they promote gut health, protect against harmful microbes, and support the immune system. Given that more than 70% of infants worldwide are not exclusively breast-fed, it is crucial to incorporate HMOs into infant formulas to ensure that they offer the same benefits as breast milk. Engineering microorganisms and using enzymes are promising solutions to produce HMOs. However, producing specific types of complex HMOs, particularly those with sialic acid modifications, remains a challenge due to the lack of suitable enzymes called sialyltransferases (STs). To address this, we developed a method for testing STs and synthesizing sialylated HMOs.