HYAL4 is an endo-hyaluronidase that degrades hyaluronan to smaller oligosaccharide fragments and exhibits chondroitin sulfate hydrolase activity 1. As a relatively distinct member of the hyaluronidase family, HYAL4 uniquely degrades chondroitin sulfate-D (2-O-, 6-O-sulphated) motifs and targets proteoglycans including serglycin, aggrecan, CD44, and sulfatase 2 1. Localized to lysosomes and cytoplasmic vesicles, HYAL4 participates in glycosaminoglycan catabolism 2. Notably, HYAL4 exists as multiple isoforms, with the V1 splice variant functioning as a chondroitinase that drives gemcitabine resistance in bladder cancer through CD44-mediated JAK2/STAT3 signaling and increased cytidine deaminase expression 3. Disease relevance includes altered HYAL4 expression in testicular, bladder, kidney, and multiple other cancers, with mutations reported across leukemia, endometrial, ovarian, colorectal, head and neck, stomach, lung, and breast cancers 1. HYAL4 expression also links to stem cell naivety and p53-negative cancer cell proliferation 1. Clinically, HYAL4-V1 levels in muscle-invasive bladder cancer specimens predict gemcitabine-cisplatin treatment response and disease-specific mortality 3, representing a potential biomarker for chemotherapy failure and therapeutic target.