Disproportionate gains made by the so-called Fang stocks may be a sign the current US bull market has peaked, Jupiter's Merlin team has warned.
Alastair Irvine, a product specialist on the high-profile fund of funds range, warned that the leading tech names in the S&P 500 had increased their value by $269bn (£207bn) between March 1 and May 9 this year.
The term Fang refers to US tech giants Facebook, Amazon, Netflix and Google - the latter of which now trades as Alphabet.
Mr Irvine said the group's rise had come at a time when the S&P 500 index had gone "absolutely nowhere". According to FE Analytics, the index was up only 0.25 per cent in local currency terms over the same period.
Year-to-date a similar disproportionate picture is displayed. The quartet have seen returns of 27, 29, 23 and 19 per cent compared to the US large cap index's rise of just 6 per cent, according to S&P Capital IQ.
"When we have a very narrow leadership of a big and broad and liquid index, it's the ringing of the bell to the Merlin team that investors need to be wary of investing in indices," Mr Irvine said.
Concerns over US stock valuations have been rising more generally. This year has seen steady incremental increases in share prices across the board - outside of the Fang group - with volatility reaching lows not seen since the early 1990s.
Commentators have stated stronger economic growth in the country as a factor behind rising valuations but others have warned of an over-reliance on soft economic metrics.
The Merlin team, led by John Chatfeild-Roberts, continued to back their North American active managers, despite concern.
In the notoriously difficult US equity market, where Fang returns have prompted assessment, the team has continued to fund the likes of Findlay Park American and M&G North American Value.
Mr Chatfeild-Roberts added: "There are various straws in the wind we might have thought were little signs that markets have gone on quite a long time.
"The US auto market is getting very nervous. Student debt in the US, their delinquencies are about 10 per cent. When we get a lot of debt it's like a balloon. Student debt is probably a significant problem."
