WE have isolated numerous variants of a mouse myeloma cell line which synthesised altered heavy immunoglobulin chains and normal light chains after treating the line with the acridine mustard ICR-191 (refs 1-4). The four types of primary variants that we found are shown in Table 1. The primary variant clones were generally stable but we did find one clone which on recloning produced subclones synthesising a variant heavy chain with molecular weight lower than that synthesised by the primary clone. This observation raised the question of whether certain primary variants might have a built-in instability and that the original mutation might be affecting a regulatory process. We were thus led to seek additional examples of spontaneous secondary variant production to see if this were a more general phenomenon. Here we report our results.
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