 Castuna is a new tool developed to control gene expression levels in mammals. It uses a novel approach based on a degrond domain that allows for precise tuning of gene expression levels. This method can be used to study how different amounts of gene product affect cellular processes such as haploinsufficiency and sex chromosome dosage compensation. This article was authored by Gemma Novello, Rutger AF Geltema and Edda G. Schultz.