SoftBank Group Corp. CEO, Masayoshi Son has resigned from the board of Chinese e-commerce giant, Alibaba Group Holding Ltd.

On resigning, Son states, he is graduating from his most successful investment by far.

The billionaire told shareholders in Tokyo he’s leaving just as Alibaba co-founder Jack Ma himself quits SoftBank’s board, but that didn’t signify any disagreements between the two billionaires. Alibaba, in which SoftBank invested is now worth $600 BN.

The pair of resignations bring to an end around a decade and a half during which the two powerful entrepreneurs — Son from Japan and Ma from China — cemented their close relationship by serving as directors of each others’ companies. Son has been on Alibaba’s board since 2005, five years after SoftBank took a major stake in the then-small Chinese online retailer. Ma came on SoftBank’s board a few years later, in 2007.

The billionaire is coming off one of the larger deals of his career, a sale of a stake in T-Mobile US Inc. that should net SoftBank as much as $20 BN. That deal along with other recent transactions meant SoftBank has now completed 80 percent of its envisioned 4.5 TN Yen unloading of assets, intended to bankroll stock buybacks and slash debt. Son told shareholders he was exploring options to complete the remaining 20 percent but didn’t elaborate.

Image Credits: Management today