668 Acta Neuropathol (2009) 118:659–671 Table 4 Demographic characteristics and classiﬁcation of patients sCJD groups MM/MV 1 MM/MV 1?2C MM/MV 2C?1 MM/MV 2C MV 2Ka VV 1 VV 2?1 VV 2 No. cases 66 63 Age at onset (years) 70.1 (48–86) 68.6 (42–89) Duration* (months) 4.0 (1–24) 4.0 (1–26) 9 65.3 (53–72) 18.1 (6–33) 4 26 5 6 39 67.8 (61–75) 65.4 (48–81) 39.3 (24–49) 69.3 (59–85) 64.5 (45–83) 20.0 (12–36) 15.8 (5–48) 15.3 (14–16) 6.5 (3.5–13) 6.3 (3–18) a Includes 2 cases with MV 2K?1 * p \ 0.001 between MM/MV 1?2C and MM/MV 2C?1 or between MM/MV 1 and MM/MV 2C?1 (Wilcoxon rank sum test) Table 5 Symptoms at disease onset (in %) in the sCJD subgroups MM/MV 1 MM/MV 1?2C MM/MV 2C?1 MM/MV 2C MV 2Ka VV 1 VV 2?1 VV 2 sCJD Groups No. of cases Cognitive Gait or limb ataxia§ Visual central Aphasia Myoclonus Psychiatric Pyramidal Sensory Other diskynesias Oculomotor 66 74 51 32 15 8 12 6 2 3 2 63 71 28 27 19 0 11 0 3 2 3 17b 76 12 35 18 0 17 0 0 6 0 9 89 0 22 33 0 0 0 0 0 0 4 100 0 0 25 0 0 0 0 0 0 26 54 85 4 4 0 0 0 12 8 0 5 100 0 0 20 0 0 0 0 0 0 6 66 100 0 0 0 0 0 0 0 0 39 25 100 0 0 0 2 0 5 0 18 a Includes 2 MV 2K?1 cases b Includes only the cases in which PrPSc type 2 was detected in at least 5 areas § p \ 0.005 between MM/MV1 and MM/MV 1?2C with more than 5 areas with type 2 and p \ 0.002 between MM/MV 1 and MM/MV 2C?1 (Fisher exact test) generated by partially digested PrPSc fragments [25], likely led other investigators to overestimate the incidence of the concurrent PrPSc types [35, 45]. Thus, the overall results on the phenomenon of the coexistence of molecular and clinico-pathological sCJD subtypes are at present incon- clusive with respect to incidence, effect on disease phenotype and criteria for disease classiﬁcation. To con- tribute to the full understanding of these issues, in the present study, we combined a systematic analysis of sev- eral brain regions in a large series of case including all codon 129 genotypes and the rarest phenotypes with the use of a reﬁned methodology for the detection of the PrPSc type concurrence, which provides good sensitivity com- bined with high speciﬁcity [25]. After screening about 4,200 samples from a largely consecutive series of 200 cases, we estimated that PrPSc types 1 and 2 coexist in about 35% of sCJD cases, which is overall consistent with ﬁgures from some of the previous studies [13, 37, 43] in which the number of cases and areas analyzed were signiﬁcantly lower. This ﬁnding supports the idea that PrPSc types co-occurrence involves a relevant but limited group of sCJD subjects and indicates that the incidence of the phenomenon had not been signiﬁcantly underestimated. 123 As far as the characteristics of the CJD population with mixed phenotypes are concerned, our data show that the PrPSc types 1 and 2 co-occur more frequently in the MM than in the MV and VV genotypes. More speciﬁcally, the large majority of sCJD cases with concurrent PrPSc types combines features of the MM and MM 2C sCJD subtypes, in variable proportions. Most commonly, in such cases, the MM1 phenotype is predominant over the MM 2C pheno- type, but the opposite situation also rarely occurs. The latter results signiﬁcantly differ from those obtained in most previous studies. Indeed, Head et al. [13] mainly found a focal type 1 co-occurrence in MM and MV sub- jects with dominant type 2, Schoch et al. [40] detected the mixed protein types mostly in MV2 cases showing the type 1 only focally in subcortical areas, and Uro-Coste et al. [43] mainly detected a random co-occurrence of type 1 in MV or VV cases with dominant type 2. Given that only our study was based on a large series of consecutive cases, we attribute such heterogeneity of previous results to case selection biases, although methodological differences may also have contributed [43]. Since subjects with mixed PrPSc types represent a signiﬁcant proportion of the sCJD population, show distinctive phenotypic features, and potentially represent a