B6d2f1

B6D2F1 purchased from Japan SLC (Hamamatsu, Shizuoka, Japan) or CLEA Tokyo, B6D2F1 mice were used for generating Ssmem1 mutant founder mice. In-house hybrid mice (C57BL/6J × 129S5/SvEvBrd) were mated with Ssmem1 heterozygous (HET) mice to expand the line. For phenotypic analysis, sexually mature male mice (6 weeks to 6 months old) were used. All mice were housed with a 12 h light cycle. All mouse experiments were performed according to the guidelines from the IACUC at Baylor College of Medicine (protocol AN-716).

ICR (Japan SLC, Shizuoka, Japan), C57BL/6 (SLC), B6D2F1 (SLC), and R6/2 transgenic mice^{19 (link)} were used. R6/2 mice carry the 5′ end of human Htt gene composed of the promoter and exon1 with expanded CAG repeats (140–147 repeats)^{19 (link)}. For superovulation, pregnant mare serum gonadotropin (PMSG) (7.5 units, ASKA Pharmaceutical, Tokyo, Japan) or CARD HyperOva (KYUDO, Saga, Japan) was injected into the abdominal cavity of female mice, followed by human chorionic gonadotropin (hCG) (7.5 units, ASKA Pharmaceutical) 48 h after PMSG or HyperOva. All animal experiments were approved by the Animal Care and Use Committee of the Research Institute for Microbial Diseases, Osaka University, Japan (approval code: H30–01-0; approval date: 4 July 2018).

B6D2F1, C57BL/6J, and ICR mice were purchased from Japan SLC or CLEA Japan. Mice were acclimated to a 12-h-light/12-h-dark cycle. All animal experiments were approved by the Animal Care and Use Committee of the Research Institute for Microbial Diseases, Osaka University, Japan (#Biken-AP-H30-01).

B6 (C57BL/6NCrSlc) and B6D2F1 ([C57BL/6NCrSlc × DBA/2CrSlc]F1) mice were purchased from Japan SLC. ICR mice were purchased from CLEA Japan Inc. Wild-derived mouse strains, CASP/1Nga (M. m. castaneus, RBRC03108, Fig. 1A) and CAST/Ei (M. m. castaneus, RBRC00733, Fig. 1B) were provided by RIKEN BRC. Female C.B-17/Icr-scid/scidJcl mice (SCID mice, CLEA Japan Inc., Tokyo, Japan) were used as the hosts for transplantation in the teratoma formation assay. Animals were provided with water and commercial laboratory mouse chow ad libitum and were housed under a controlled lighting condition (daily light from 07:00 to 21:00). For euthanasia, cervical dislocation was performed by trained individuals. Prior to the operation, mice were deeply anesthetized by intraperitoneal injection of tribromoethyl alcohol (avertin) or a mixture of three types of anesthetic agents (medetomidine, midazolam and butorphanol). They were maintained under specific pathogen-free conditions. The care and use of animals in this study were performed according to the guidelines for the use and maintenance of experimental animals from the Japanese Ministry of Environment. All animal experiments included in this study were approved by the Institutional Animal Care and Use Committee of RIKEN Tsukuba Branch. This study is reported in accordance with ARRIVE guidelines (https://arriveguidelines.org).

B6D2F1 and ICR mice were purchased from Japan SLC (Shizuoka, Japan). B6D2F1 females were consecutively injected with pregnant mare serum gonadotropin (PMSG; ASKA Pharmaceutical, Tokyo, Japan) and human chorionic gonadotropin (hCG; ASKA Pharmaceutical) at a 46–48 h interval and then crossed with B6D2F1 males to collect fertilized eggs. Pseudopregnant mice were prepared by mating ICR females with vasectomized ICR males. All animal care and experiments were carried out in accordance with the Guidelines of Animal Experiments of Kitasato University and approved by the Institutional Animal Care and Use Committee of Kitasato University.