Fig. 3. Development of cerebellar syndrome in ageing Prnp^–/– Zürich II mice. (A) Footprints of wild-type (Prnp^+/+), Zürich I (Prnp^0/0 ZHI) and Zürich II (Prnp^–/– ZHII) mice at 11–12 months of age. The hind feet were dipped in ink and the animals prompted to walk on filter paper. Zürich II mice are unable to walk straight and show a generally reduced step length compared with Zürich I and wild-type animals. (B) Time-course of the cerebellar symptoms (intention tremor and trembling gait) in Zürich II (ZH II) mice (n = 63), F1 of Zürich I × Zürich II (ZH I/ZH II) mice (n = 13), Zürich II mice transgenic for cosmid Cos-tTA (Cos-tTA/ZH II) (n = 23) or a Tga20 allele (Tga20/ZH II) (n = 17) and Zürich II mice containing a single wild-type allele (Prnp^+/– ZH II) (n = 3 for observation over 20 months; n = 19 for observation over 7–8 months).