erictopol.substack.com/p/your-thymus-and-your-healthspan

This appears to conflate two different molecules discussed in the same paragraph.

In the cited Nature Aging paper, β-klotho is described as the obligate co-receptor for FGF21, not for FOXN1. The paper specifically states: "Ablation of β-klotho, the obligatory co-receptor for FGF21 in Foxn1+ TECs, accelerated thymic aging." FOXN1 is a transcription factor important for thymic epithelial cells; it is not the ligand for which β-klotho serves as a co-receptor.

So the article's wording is incorrect: β-klotho should be linked to FGF21, not FOXN1.