PTDSS1 and primary bone dysplasia with increased bone density: In mice, PTDSS1 and PTDSS2 can compensate for each other,25, 26 but simultaneous disruption of both genes is lethal, implying that PTDSS is absolutely required for viability.25 Dominant heterozygous mutations in PTDSS1, leading to a gain in enzyme function, have been associated to a syndrome of sclerosing bone dysplasia, intellectual disability and distinct craniofacial, dental, cutaneous (cutis laxa) and distal‐limb anomalies (Lenz‐Majewski syndrome [MIM #151050]).27