Вероятно, "бета-" здесь указывает на один из возможных участков hTERT, в котором может происходить делеция, потому что встречается упоминание альфа- и бета-делеций вот такого вида:

https://www.ncbi.nlm.nih.gov/pubmed/11191110
An alternate splicing variant of the human telomerase catalytic subunit inhibits telomerase activity.
Yi X1, White DM, Aisner DL, Baur JA, Wright WE, Shay JW.
The telomerase transcript has been shown to have at least six alternate splicing sites (four insertion sites and two deletion sites), and variants containing both or either of the deletion sites are present during development and in a panel of cancer cell lines we surveyed. One deletion (beta site) and all four insertions cause premature translation terminations, whereas the other deletion (alpha site) is 36 bp and lies within reverse transcriptase (RT) motif A, suggesting that this deletion variant may be a candidate as a dominant-negative inhibitor of telomerase. We have cloned three alternately spliced hTERT variants that contain the alpha, beta or both alpha and beta deletion sites.