Protein–protein interaction (PPI) network analysis of the top 100 DEPs related to yak HF growth and regression revealed that, in addition to various Krt proteins and cell junction proteins such as DSG1, DSG2, DSP, DSC1, and CLDN1 directly associated with hair structure, several lipid-related proteins including ACSL1, C3, APOH, AUP1, and FASN served as core nodes in the interaction network (Figure 2G,H). This evidence concerns the gene CLDN1 and hydrops fetalis.