Telomeres are DNA repeats that cap the chromosome ends. Each time a cell divides, its telomeres become shorter. This shortening is a key driver of senescence. Cancers evade this by adopting a telomere maintenance mechanism, via activation of either telomerase enzyme or ALT (Alternative Telomere Lengthening) pathway. ALT pathway is used in 15% of human cancers including gliomas, osteosarcomas and soft-tissue sarcomas. DNA sequencing studies have identified frequent ATRX mutations in human ALT cancers. However, loss of ATRX alone is insufficient to induce ALT, and this means other ‘missing factors’ are needed to turn on the ALT pathway. We show that H3.3 G34R (Gly34 to Arg) and IDH1 R132H (Isocitrate dehydrogenase 1; Arg132 to His) are two such 'missing factors' that act with ATRX mutations to activate the ALT pathway in gliomas. Both mutations inhibit H3K9/K36 KDM4 demethylases and KDM4B is the key demethylase inactivated in ALT cancers. Primary cell models inactivated for ATRX, KDM4B (by H3.3 G34R or IDH1 R132H), TERT (telomerase) and TP53 tumour suppressor develop ALT features. In contrast, KDM4B expression disrupts ALT features and kills human ALT cancer cells. We also show evidence that KDM4B binding at telomeres is controlled by H3.3 serine 31 phosphorylation (H3.3 S31Ph), and H3.3S31Ph is critical factor for maintaining telomere and genome integrity. In summary, our studies showed that KDM4B is a novel telomere regulator, and KDM4B inactivation is the ‘missing factor’, acting together with ATRX mutations, to activate the ALT pathway in cancers.