For eight of the nine haplotypes with a significant effect on calving rate (see Mapping section), Fritz et al. (2013) searched for causal mutations via whole-genome sequence data from 25 Holstein, 11 Montbéliarde and nine Normande bulls which had made major contributions to their breed. Specifically, they filtered "for mutations that were (a) located at+or –6 Mb from the detected haplotype (b) carried in the heterozygous state by the carrier bulls and (c) absent from the non carrier bulls from the three breeds" and then examined identified polymorphisms for their likely effect on protein structure and function. For MH2, Fritz et al. (2013) provided strong evidence for a candidate causal mutation, namely a nonsense mutation (g.28879810C>T; UMD 3.1 genome assembly) in the SLC37A2 gene (which solute carrier family 37, member 2), leading to p.R12X. Reinartz and Distl (2016) reported homozygosity of this mutant in an aborted Vorderwald x Montbéliarde crossbred foetus inbred to Montbéliarde bulls. The mutant occurs at a frequency of around 5% in Vorderwald cattle with Montbéliarde ancestry but is absent from Vorderwald cattle with no Montbéliarde ancerstry.
