Classic HCL and HCL-V are genetically distinct. In classic HCL, there's usually a mutation on the BRAF V600E gene, but this mutation isn’t there for people with HCL-V. Instead, HCL-V typically has a mutation on the MAP2K1 gene. This can lead to the diseases behaving differently. For example, people with HCL often have monocytopenia as a symptom, whereas people with HCL-V usually don’t. For this reason, classic HCL treatments that target the mutated BRAF V600E gene don’t work as a treatment for HCL-V.