This study looks at how safe and effective a medicine called Xromi® (hydroxycarbamide) is for young children with sickle cell disease (SCD), which is a blood disorder that can cause pain and other health problems. The study checks both children who take Xromi® and those who don't. It lasts for 24 months (about 2 years) from when they start treatment. It's a "non-interventional" study, meaning no new treatments are introduced, just observations on existing ones.

**Hydroxycarbamide** is a medicine sometimes used to treat SCD by helping to prevent painful episodes and other complications. **Adverse events of special interest (AESI)** are specific side effects that researchers are watching closely. The study will compare how often these happen in children taking Xromi® versus those who aren't.

• **Duration**: 24 months of follow-up.
• **Observation**: No new treatments, just comparing existing conditions.
• **Location**: Conducted in the UK and Germany, possibly other European countries.