library_construction_protocol: Human intercostal muscle biopsies (inner part between the second and third ribs) were collected with consent from deceased transplant organ donors by the Cambridge Biorepository for Translational Medicine, Cambridge, UK (CBTM), immediately placed in HypoThermosol FRS preservation solution and shipped to Sanger Institute for processing.  Skeletal muscle tissue was processed according to the following protocol for single-nucleus isolation: https://www.protocols.io/view/nuclei-isolation-from-human-skeletal-muscle-kqdg34pqel25/v1. Briefly, muscle tissue was grounded using a dounce homogenizer, lysed in the nuclei lysis buffer and, finally, Percoll gradient was used to separate intact nuclei for loading and cell debris. Cell or nuclei suspension were  loaded on the Chromium Controller (10X Genomics) with a targeted cell or nuclei recovery of 8000.  10X Genomics Single Cell 3â€™ v2 or v3 Reagent Kit was used to create GEM, perform cDNA synthesis and generate sequencing libraries according to standard instructions.