DNAJB13 is a type II HSP40 co-chaperone that plays essential roles in sperm flagellar function and ciliary motility 1. The protein localizes to sperm flagella and is an axoneme-associated component, being present along the entire length of the flagellum but absent from structures lacking the axoneme 2. During spermiogenesis, DNAJB13 is expressed in spermatid cytoplasm from steps 2-3 onward and associates spatiotemporally with the annulus, a septin-based structure connecting the midpiece and principal piece of sperm flagella 3. DNAJB13 directly interacts with SEPT4 and may be involved in assembling and positioning the annulus structure 3. The protein also interacts with radial spoke head proteins RSPH1, RSPH9, and helps regulate central pair stability in flagella 4. Mutations in DNAJB13 cause primary ciliary dyskinesia and male infertility, with affected individuals showing sperm tail defects, immotile spermatozoa, and respiratory infections 156. Intracytoplasmic sperm injection (ICSI) has shown successful outcomes for patients with DNAJB13 mutations 56. The protein appears to function specifically in sperm flagella rather than respiratory cilia 4.