SPINK5 and ichthyosis: Indeed, SPINK5 knockout mice or mice with a premature stop codon in SPINK5—both are mouse models of Netherton syndrome, a severe ichthyosis due to null mutations in SPINK5—display increased amounts of mature filaggrin and reduced amounts of profilaggrin, indicating that proteolytic processing of profilaggrin is enhanced [39,40].