AbstractBackgroundX‐Linked dystonia‐parkinsonism (XDP) is an adult‐onset neurodegenerative disorder characterized by rapidly progressive dystonia and parkinsonism. Mosaic Divergent Repeat Interruptions affecting motif Length and Sequence (mDRILS) were recently found within the TAF1 SVA repeat tract and were shown to associate with repeat stability and age at onset in XDP, specifically the AGGG [5′‐SINE‐VNTR‐Alu(AGAGGG)2AGGG(AGAGGG)n] mDRILS.ObjectiveThis study aimed to investigate the stability of mDRILS frequencies and stability of (AGAGGG)n repeat length during transmission in parent–offspring pairs.MethodsFifty‐six families (n = 130) were investigated for generational transmission of repeat length and mDRILS. The mDRILS stability of 16 individuals was assessed at two sampling points 1 year apart. DNA was sequenced with long‐read technologies after long‐range polymerase chain reaction amplification of the TAF1 SVA. Repeat number and mDRILS were detected with Noise‐Cancelling Repeat Finder (NCRF).ResultsWhen comparing the repeat domain, 51 of 65 children had either contractions or expansions of the repeat length. The AGGG frequency remained stable across generations at 0.074 (IQR: 0.069–0.078) (z = −0.526; P = 0.599). However, the median AGGG frequency in children with an expansion (0.072 [IQR: 0.066–0.076]) was lower compared with children with retention or contraction (0.080 [IQR: 0.073–0.083]) (z = −0.007; P = 0.003). In a logistic regression model, the AGGG frequency predicted the outcome of either expansion or retention/contraction when including repeat number and sex as covariates (β = 80.7; z‐score = 2.63; P = 0.0085). The AGGG frequency varied slightly over 1 year (0.070 [IQR: 0.063–0.080] to 0.073 [IQR: 0.069–0.078]).ConclusionsOur results show that a higher AGGG frequency may stabilize repeats across generations. This highlights the importance of further investigating mDRILS as a disease‐modifying factor with generational differences. © 2024 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.