KRT15 and hydrops fetalis: Using keratin 15 (K15) expression and K15 promoter-driven green fluorescent protein (GFP) expression as a system for assessing human epithelial HF stem cell functions in situ and in vitro[35], [37], [38], we also explored whether spermidine alters human HF epithelial stem cell clonogenicity, whose modulation by polyamines is as yet unknown [17], [18], [38], [39].